SPARK
Speaking generally, a spark is a mad scientist. Megalomania, cackling laughter, delving in Things Man Was Not Meant To Know; all of these are easily recognized as bona fide spark traits, although, as with everything, there's more to it.
While ultimately the label of "mad scientist" isn't incorrect, it is a bit too specific. It suggests a long career, a degree, the image of the gentleman scientist doing tests in his basement because the world just isn't ready for his genius; in short, it's a learned trait, not an inhereted one. The spark, meanwhile, is an inhereted ability, though it doesn't tend to become visible until the spark is in his/her teens, an ordeal called "breaking through." Many sparks end up dead this way, as breakthrough devices are generally violently destructive; either they end up accidentally killing themselves with their creations, or they find themselves at the mercy of townspeople who don't take too kindly to a rogue clank tearing up their village.
All sparks have a theme to their creations, a general style that they follow, like a painter. From the comic, for example, Agatha specializes in clanks and weapons, typically with a somewhat musical theme, a reference to the humming she utilizes to help her think. She is not all that familiar with biology, though; it's unlikely that she will be producing any constructs any time soon. When a friend of hers ended up wounded and sick, she patched him up a bit and then left to find another spark to take care of it because she wasn't sure what she was doing.
When a spark really gets into the work s/he is doing, or if s/he gets particularly angry, they enter a state of mind referred to as the "madness place." It's characterized by intense concentration, ranting, cackling, breaking the laws of nature, and the abandoning of things as trivial as common sense. This is referred to as "sparking," and occurs in stages.
The first stage is marked by heightened concentration and focus, but has not yet progressed into a manic state. Typically when you see this state it's because the spark is progressing to the second.
The second stage is the most commonly pictured when one thinks "spark." Manic ranting, not-quite-sane laughter, delusions of grandeur, and either extreme happinessor extreme anger. A spark may also switch between these two from one second to the next.
The third stage is solely one of rage. When a spark needs to be threatening, or indeed if something has simply pissed them off, this is the mode you'll find them in.
For more information, you can visit the Wiki entries on the spark and the madness place. Most of the information here is paraphrased and trimmed from these pages.