From another list I'm on:

 http://tweety.bowlofmice.com/tweety/troll.html
 
Define Troll, Somebody
 
What exactly is a troll? In fantasy novels, it's a large, somewhat stupid creature that rarely bathes with poor dietary habits. In Everquest, it's a big cuddly critter easily mistaken for a sea turtle roleplayed by people who are training for a job running a Muppet. In nursery rhymes, it's a bridge dwelling monster easily outwitted by goats.

And on message boards, it is a faceless demon with a sign reading "Ban Me" taped to its ass. Everyone knows you shouldn't feed the troll, and everyone does it. If in real life, you meet a troll, you ignore the moron and order another beer. But on a message board, everyone feels some bizarre compulsion to respond, to edify, to enlighten. Six dozen people take turns logically responding to the troll, and the troll responds to every post with some long drawn out manifesto from hell. When semantic nulls are removed from these documents, they boil down to "nuh uh, you stupid poo poo head." Anyone who has ever attempted to argue with a two year old or a congressman knows how futile the debate has become when the poo poo head pejorative has been unleashed. The only response to "nuh uh" is "shut up, you mouth breathing bottle baby before I go back in time and give your father a condom" and then everyone feels vaguely unclean.