An emitter is a collection of properties that determine how particles look and behave. But that sounds like an abstract science text book description.
An emitter in particleIllusion is really just a visual effect that you can customize. How each emitter looks varies depending on a number of things. You change the speed that its particles move at, how they move, and what they look like to design a new effect.
There are about 1500 emitters that you can download from the wondertouch download page with the full version. The demo has something like 60 emitters.