A spark is the lifeforce, or soul, of a Transformer. Each spark is part of the essence of Primus himself.
The spark is a physical entity which is safeguarded inside a Transformer body. If something harms the spark beyond repair, it is "extinguished" and returns to the Allspark (or sometimes the Well of All Sparks). The Transformer is then considered dead. A Transformer is able to survive an enormous amount of physical damage. As long as the spark is intact, it is possible to repair or rebuild the body and restore the spark to it.