A spark is basically the soul of a Transformer, it is what truly separates them from normal robots.
Types of Sparks:
Spark: The basic kind most Transformers have
Dark Spark: A rare mutation that causes malevolent behavior in the Transformers who have one, this is not a common occurance, even most Decepticons don't have Dark Sparks... And some believe the sadistic behavior of the Autobot Blades may indicate he has one. The Decepticon Skyquake has one, and the Spark of Jazz was once corrupted and turned into a dark spark.
Immortal Spark: Only Starscream is known to have one, though the Maximal/Predacon version of Rampage has an "artificial" immortal spark.