The geth were created by the quarians as a labour force. They were designed as VIs, as advanced as possible while remaining non-sentient. They were also designed to operate more efficiently when networked together. Unfortunately, this feature was the quarians’ undoing. Geth programs were indeed non-sentient individually, but slowly gained sentience through the massive geth network. Eventually, they started asking the quarians questions only sentient beings would think to ask; in one notable instance, a domestic geth unit asked its owner if it had a soul. Alarmed at this, the quarians decided it would be best to shut down all geth before they conceived of revolt. The attempt failed, and a war began between the geth and the quarians, which geth afterwards referred to as the Morning War.
At first, the geth did not respond to the termination order with violence; it was only after panicked quarians fired upon them that the geth thought to pick up weapons and defend themselves. Even after this, some geth remained loyal to their creators and put themselves in harm's way to protect geth sympathizers from persecution; likewise, there were quarians who did not feel the geth deserved to die. However, as time went on, the geth sympathizers were outnumbered, and the war continued, eventually seeing the geth gain the upper hand. The war ended when the surviving quarians evacuated their home world and colonies in the Perseus Veil. Unknown to the quarians themselves, the geth actually allowed them to leave; unsure of the repercussions of eradicating an entire species-namely their own creators- and having decided that the quarians were now too weak to be a threat, the geth decided to draw back their forces so that the surviving quarians could flee. The fleet of quarian ships that escaped the Veil became known as the Migrant Fleet, and has been roaming the galaxy ever since.
Little is known about the geth in the time between the Morning War and the present. The geth did not repopulate the now barren quarian worlds, instead choosing to exist in the computer hubs aboard massive space stations and extract needed resources from asteroids. They adopted an extremely isolationist attitude - any ships that ventured into geth space were immediately attacked and destroyed. While they prevented any contact by other races with themselves, the geth monitored communications and the extranet. The geth continued development of new technology and variations of mobile platforms, separating their technology base from the rest of the galaxy. They obtained an ultimate goal in this time period - to create a Dyson Sphere-like object, which could house every single geth program.
Approximately three centuries after the Morning War, the geth were approached by the Reaper Nazara, also called Sovereign. It offered them technology that would aid them in achieving their goal, in exchange for their assistance in capturing the Citadel and letting the Reaper invasion begin. The majority of the geth dismissed the offer, deeming it better to accomplish their goal with their own technology rather than be dependent on another race’s technology. These geth discarded what they called the "superstitious title" of the Reapers, and simply called them the Old Machines. A small percentage of geth, however, accepted the Reaper’s help. Henceforth these geth were referred to by the mainstream geth as "heretics". They were allowed to peacefully leave the main geth network, and aid Nazara and its turian agent, Saren. The heretics came to revere Nazara as a god, the pinnacle of synthetic evolution. Unknown to them, Nazara was repulsed by the heretics' worship of it, though it recognised their value as tools to facilitate its goals. They aided Nazara and Saren in many engagements, such as the attack on Eden Prime, and the Battle of the Citadel. After Nazara was destroyed in the latter battle, the heretics lost much of their menace, and their operations outside of the Perseus Veil were quickly mopped up by Council forces. The heretics did maintain operation of a large space station within the Terminus Systems, located in deep space between stars.
Using Sovereign's influence over them, the rogue Spectre Saren Arterius commanded an army of geth in an effort to take over the Citadel and release the Reapers from dark space. The geth formed the bulk of Saren's forces, following him because they believed he had the means to find the Conduit and bring back their "gods". In addition to providing ground troops, the geth also crewed (or tended to) Sovereign.
However, the geth did not realise Sovereign was actually insulted by their "pitiful devotions" and saw them merely as tools. After the Protheans broke Sovereign's hold over the keepers, and the keepers evolved so that they only accepted commands from the Citadel, the Reaper realized organic races were difficult to control. It found the geth to be suitable replacements as servants, and exploited their religious beliefs. Saren claimed that, although they were viewed disparagingly by Sovereign, the geth were valuable as tools, and would therefore survive the Reaper invasion because they were useful. The example of the geth inspired Saren to prove organic races could also be useful to the Reapers. He hoped that, instead of harvesting them, the Reapers would spare the organic races of the galaxy, even if that meant they would only survive as slaves.
Saren's plans were foiled by Commander Shepard and the crew of the Normandy, and Sovereign was destroyed in the battle over the Citadel. The remaining geth that were not destroyed in the assault on the Citadel continued to put up resistance and were systematically hunted down by an emergency Citadel coalition force, headed by the Alliance.
Mass Effect 2
The true geth became interested in Commander Shepard, as Shepard had defeated Nazara and the heretics. They commissioned a unique mobile platform, holding 1183 programs rather than the normal hundred or so, to operate independently and go looking for Shepard. This geth retraced Shepard’s footsteps, from Eden Prime to Virmire to Ilos. During this time, the geth sustained considerable damage, and used a discarded piece of Shepard’s N7 armor it found to partially repair the damage. It tracked Shepard all the way to the crash site of the SSV Normandy, where the trail went cold—Shepard had died there. The geth was reassigned to a different task.
Though the heretics had faced a major setback with the defeat of Nazara, they did not rejoin the main geth. Rather, they continued development of a special virus which Nazara had given them, in an unfinished version. This virus could alter the result of a fundamental calculation in geth processes, radically changing their logic and reasoning. Through this change, the virus could force the true geth to accept the heretics’ conclusion, or it could force the heretics to accept the true geth’s conclusion. The true geth understood the threat, and sought a way to eliminate the virus. However, the virus was stored on a Reaper data core, which the true geth could not access. To gain an understanding of Reaper technology, the unique mobile platform was sent to a derelict Reaper. Aboard the Reaper, it encountered Commander Shepard, who was on a mission to recover an IFF in order to safely use the Omega-4 mass relay. The mobile platform was brought back to the Normandy SR-2, where it accepted the designation "Legion". Shepard aided Legion in its mission, going to the Heretic Station. Once there, Legion realized that the virus was ready, and could be used to rewrite the heretics’ behavior. Unable to form a consensus on the matter, Legion left the decision to Shepard—destruction of the station, or the rewriting of the heretics. Either choice constituted a crippling blow to the heretics.
Mass Effect 3
It is revealed during Mass Effect 3 that the quarians have declared war on the geth. They have successfully managed to push them back to their home system, where the quarian homeworld Rannoch is located.
After Legion's return to the geth consensus and revealed its evidence on the imminent return of the Reapers, the geth began preparing for war. However, while building their mega-structure, the quarian flotilla attacked and destroyed it. A significant number of geth programs were installed at the time the quarians began bombing, and due to a lack of surplus server hardware not all of the programs could be saved. With the loss of so many programs, their intelligence dimmed and survival took precedent among the consensus, causing the geth to choose to make a deal with the Reapers, allowing themselves to be controlled by Reaper code in order to become more effective fighters, believing the cost of their free will an acceptable price to avoid extinction. They were then able to bring the fight to the quarians, trapping their civilian ships and destroying much of their fleet.
Commander Shepard arrived and was given the task of infiltrating the geth dreadnought from which the controlling signal was being broadcast, in order to return the geth to a defeatable state. After fighting through a large number of geth platforms, Shepard arrives at the drive core to find either Legion, or a geth VI trapped inside a piece of Reaper tech, from which the signal came. The Commander deactivates the device and frees the unit that had been contained within.
As a gesture of goodwill, the geth unit deactivated the ship's weapons and barriers. However, the quarian heavy fleet took advantage of this and began to fire on the dreadnought while Shepard and their crew were still on board. Shepard managed to escape despite this, and the geth were now at a disadvantage.
Legion, or the Geth VI, went on to aid Shepard in the disabling of geth fighter squadrons targeting the quarians' liveships and then attacking a Reaper base on Rannoch, later revealed to be an actual Reaper, that was still able to transmit the code to the rest of the geth.
After the Reaper on Rannoch dies, the quarians attack the geth while they are still disorganized. Depending on Shepard's choices either Legion/Geth VI will upload the remainder of the Reaper code, thus allowing each geth program to achieve true individual consciousness and the power to resist and obliterate the quarians, the upload is interrupted and the geth eliminated, or Shepard convinces the quarians to stand down and the two sides hesitantly make peace.
If Shepard allowed the geth to achieve true intelligence, they will provide assistance with the War Effort. The Systems Alliance and the Turian Hierarchy receive geth support with open arms, although there is still a grudge between them. If peace was made with the quarians, it is revealed that the geth are able to coexist with them on Rannoch, helping setting up housing and uploading into quarian suits, mimicking infections that, in time, will help the quarians to boost their immune system.
If at the end of the game, Shepard chooses to use the Crucible to destroy the Reapers, all synthetic life in the galaxy including the geth will be destroyed. If Shepard chooses to uses to the Crucible to take control of the Reapers or merge all synthetic and organic life, the geth will help to rebuild Rannoch.
The geth are reclusive and secretive. This is partly due to their synthetic nature: They have no need to interact with other races because they do not share the same goals, needs or instincts as organic species. As machines, comfort is also not a concern for them, something that is reflected in elements of geth ship and station design (e.g., minimal gravity, efficient use of space, function over form, lack of atmosphere, and absence of climate control).
According to Legion, geth do not actually live on any of the quarian planets they conquered, serving merely as caretakers for them instead. They find it more efficient to live on space stations and draw resources from asteroids, though they maintain mobile platforms on the worlds to clear rubble and toxins left by the Morning War. Legion likens this behavior to the human tendency to establish cemeteries and memorials to commemorate dead persons and tragic events. However, since geth do not "die" in any traditional sense (upon the destruction of a geth platform, its programs are simply transmitted to the nearest available platform) and so have no real losses to mourn from the Morning War, Legion posits that they instead clean and maintain the quarian worlds out of respect for their quarian creators who died in the conflict and in preparation for the eventuality of their return.
The only geth who have interacted significantly with organics aside from the Legion platform are a sect that Legion refers to as the "heretics". They are a small, radical group of geth who worship a hyper-advanced but long-vanished machine race called the Reapers, whom they see as the pinnacle of non-organic evolution. It is these geth who attacked Eden Prime and the Citadel. This group is estimated to consist of about five percent of the total geth population. According to Legion, the mainstream geth are content to stay separate from the remainder of Citadel space to "build their own future," and claim that all sentient creatures should have the ability to "self-determinate", strictly adhering to a policy of non-interventionism in respect to the affairs and development of other races. While they have made attempts to observe and understand organics (an effort embodied by the unique platform known as Legion), the geth primarily seek the peaceful advancement of their own race independent of the influence of the rest of galactic society and believe every sentient species should be able to do the same, an attitude which shows that the geth are significantly less hostile than the galactic community initially believed.
Depending on the context, the term "geth" can refer to an individual program, mobile platforms which house a number of individual programs, or the entire collection of all programs. Organics generally refer to each mobile platform as a geth. However, the geth do not share the concept of individuality, and think in terms of the entire collective.
Legion also gives insight into the psychology and society of the geth. Geth psychology is completely alien to organics. They do not value individuality, preferring to share all memories and thoughts of all programs. This means geth cannot and do not wish to hide their thoughts, even those that do not follow the norm, with their reasoning for differing thoughts being apparent. Legion often uses the term "we" instead of "I", since it is both part of the geth society as a whole and is also a gestalt of over a thousand geth programs.
More importantly, deceit, manipulation and lying are impossible among the geth. No concepts of vulnerability or privacy exist; geth are completely honest with each other about their thoughts and their reasons for those thoughts. This means there is a great deal of understanding amongst geth, with every geth being part of the consciousness of every other geth. A possible consequence of this type of networked intelligence is theorized by EDI to be why the geth were able to turn on the quarians, preventing them from developing preferences and devaluing the significance of an individual life.
Geth have no government or leaders; the geth use FTL communications to "build a consensus," a completely Consensus Democratic method in which every geth program makes a choice on any matter. Even within Legion itself, consensus must be reached among its many programs before decisions can be made.
While the geth are incapable of deceit amongst themselves, they are not above using deceit to study organic behavior. One example is the geth introducing a falsified report on the extranet detailing a constellation resembling a salarian goddess seen from the batarian homeworld, which some salarians immediately believed to be proof of the goddess' existence. The experiment ended when a salarian cult attempted to buy the rights to the stars, only to find that they did not exist.
There is some question of whether geth personalities remain stable over time. When an artificial intelligence is transferred to a new blue box, a completely new personality is created. Geth frequently download into different hardware according to need, from starships to the "mobile platforms" that Shepard is familiar with. It is unknown whether new personalities result from these downloads, or whether it matters to the geth.
According to Legion, the geth that served Sovereign were actually a splinter faction, and the much larger geth collective opposes the Reapers. Legion is part of this larger faction; they refer to the Reapers as "Old Machines" and Sovereign as Nazara. Legion explains that other geth have no hostility towards Commander Shepard or even the quarians. While the geth heretics that serve the Reapers want the Reapers to give them a future, Legion and like-minded geth want to make their own future.
Despite this difference, the baseline geth respected the heretics' decision and did not label it as "wrong". Using another analogy, Legion explains that in simple terms, "Heretics say, one is less than two. Geth say, two is less than three." The heretics were allowed to leave peacefully, to join their god and its prophet.
It was therefore a shock to Legion when the heretics grew deceitful against the baseline geth. The heretics planted run-times within geth networks to spy on them, and obtained a Reaper virus from Sovereign capable of changing a geth's judgement, introducing a subtle math error which would eventually compel them to believe that worshipping the Reapers was correct. If Legion is allowed to join the Shepard's team, they can go to Heretic Station in The Sea of Storms to stop the virus. Legion suggests that the virus could be re-purposed to reprogram the heretics into rejoining the collective or destroy them, thus depriving the Reapers of useful servants. Legion, however, cannot reach a "consensus" among its programs and defers to Shepard's judgment, insisting Shepard make the decision because the Commander has fought the heretics and the "Old Machines". This gives Shepard a perspective the baseline geth lack.
Should Shepard state that rewriting the geth is morally acceptable as geth are machines, Legion will concur with the Commander's logic. If Shepard's team member expresses surprise at Legion for this, it will respond that to believe every being shares your views and abides by your morality is arrogant and racist. Ultimately, Shepard can choose whether to destroy the heretic geth, simply removing the threat, or reprogram the heretics to no longer follow Sovereign and return to the geth.
The long-term goal of the geth is the construction of a "mega-structure", a massive mainframe capable of simultaneously housing every existent geth program, thereby maximizing their collective processing capacity. As of 2185, the geth have already been in the process of constructing the mega-structure for 264 years. Legion suggests the closest conceptual analogue is a Dyson sphere and, as it poignantly states "no geth will be alone when it is done." It believes that Sovereign promised the heretics a Reaper body to serve as such a mega-structure.
Notably, however, Legion considers this arrangement a Faustian bargain. The position of the baseline geth is that even if they share an ultimate goal with the heretics, self-determination toward that end has intrinsic value. In other words, the journey is as important as the destination. The heretics are to that effect consequentialistic, where the "true geth" are deontological. Something ineffable would be lost if the geth—and indeed, all galactic civilization—were merely to evolve along the rigid lines arranged by the Reapers.
The key element of geth warfare is surprise. Their sudden and unexpected return from beyond the Veil after three centuries was typical. Shepard describes the geth as perfect ambushers—"they don't move, they don't make noise, they don't even breathe." Their freedom from the need to eat or sleep allows the geth to leave dormant garrison units in ambush at key positions, and the fact they don't feel pain allows them to drop units directly onto the battlefield from high altitudes. Geth can even be packed tightly into crates and left in storage, shutting down their power sources to stay undetected. A tactic particularly favoured by the geth is to set "distress signals" and then ambush the rescuers; they also turned the freighter MSV Cornucopia into a drifting trap.
In combat, geth units show little sense of self-preservation. Because geth programs can share memories and files with extreme ease, the experiences of individual geth programs are not lost when a platform is destroyed and archived versions of programs can be downloaded into new bodies. As geth programs are functionally immortal, they place little value on platforms and will expend thousands of units to take an enemy position, which was commonly experienced by the quarians during the Morning War. For this reason, geth are willing to engage in seemingly-suicidal actions like boarding a ship, engineering it to crash, and then remaining on board to ensure the ship goes down.
Heretic geth use psychological warfare on the battlefield with dragon's teeth, a Reaper implement. These implements pierce the bodies of dead soldiers, turning muscle tissue into synthetic material, creating a zombie-like monster called a Husk. This severely lowers morale, as their enemy now has to kill fallen comrades.
When geth shut down, they fry their memory cores as a defensive measure, which is probably why geth haven't been successfully captured for study. However, Tali'Zorah nar Rayya was able to retrieve some of the data cache from a deactivated geth, using her quarian tech expertise and quick decisiveness.
Geth weapons and armor are of extremely high quality but are difficult to find—a Geth Armory license allows Shepard to buy them. Their weapons are pulse based. These weapons are among the most powerful in the game and ideal for Shepard's team.
Best size estimates on a single geth fleet range from 5,000 to 10,000 ships with unknown armament. These fleets include dreadnoughts, dropships, and fighters. Since the geth are not limited by the demands for rest, wages, or autonomy that organic workforces require, they are limited only by time and raw materials in what they can construct, allowing for the creation of such massive fleets. In addition, the geth are not restricted by the Treaty of Farixen and other Citadel regulations in building their ships. Geth dreadnoughts can exceed the size of Alliance dreadnoughts by as much as 30%, and utilize more expensive and energy-demanding ultraviolet antiship lasers as opposed to the cheaper infrared antiship lasers employed by most organic built ships.
Physically, the humanoid geth resemble quarians—their hands, head shape and legs are similar—which is probably a holdover from their origins. A common design feature among geth is a single brightly glowing photoreceptor, causing some to refer to them as "flashlight heads". Geth are primarily composed of two materials: a flexible but durable outer shell, and a kind of synthetic muscle tissue that gives Geth Hoppers their incredible agility. It is actually possible for this synthetic muscle to be grafted to organic tissue (Saren Arterius has a grafted geth arm). Geth "bleed" a white conductive fluid when shot, but they don't have any internal organs or nervous system, so the geth do not feel hunger or pain. The geth are also seen using omni-tools, and have seemingly modified them to heal synthetic wounds; likewise, some geth facilities contain first-aid kits, implying that their synthetic "tissues" can be repaired using conventional medical technology.
Part of the geth's success is due to their neural network. Effectively, they "share" their processing power, distributing low-level processes like motor control and visual identification to free up bandwidth for higher reasoning and complex thought. Geth can't share sensory data—they aren't a hive mind like the rachni—but in large groups they have more to think with. An individual geth has only a basic intelligence on par with animal instincts, but in groups they can reason, analyze situations, and make tactical decisions as well as any of the organic races. An exception would appear to be Legion, a geth specifically designed to operate as an autonomous sentient unit outside the Perseus Veil. Legion possesses over 11 times as many geth programs as a standard geth platform and can function intellectually on its own.
According to Legion, who may join the squad during Mass Effect 2, each geth is made up of hundreds of programs equivalent to VIs, all operating in parallel with one another to form a kind of emergent intelligence described by EDI as "a thousand voices talking at once". An individual geth is thus more of a "mobile platform" than an actual body; the programs that make up its consciousness are constantly being transferred and downloaded; the mind operating one of these "mobile platforms" might just as easily inhabit a starship body should it need to. Most of the time geth programs can be found residing in server hubs, which function as something akin to the organic equivalent of a city, and can run millions of geth in communion.
As all geth are networked to each other, they may communicate their exact thoughts and ideas at the speed of light. They find organic methods of communication, such as body language and spoken word, to be largely inefficient; the geth are able to communicate their thoughts flawlessly without any fear of misinterpretation. Because of this they have no true form of government and no system of rank. When a matter must be decided upon, the geth communicate all viewpoints of a situation and a consensus is made, the decision being whatever benefits the geth as a whole the greatest.
Over time, the geth have evolved into numerous sub-forms — from the diminutive but highly agile Geth Hoppers, to the gigantic, lumbering Geth Armatures. The geth also utilize turrets and drones (rocket, assault, recon and repair drones, specifically). It isn't clear whether these are also AIs — in the same way Geth Armatures are not just tanks but sentient machines capable of learning and problem solving — or simply controlled by the geth.
It should be stressed, however, that practically all geth encountered outside the veil likely belong to a violent faction of the geth and will react with hostility to any organic not working for the Reapers.