In my understanding, someone who posts on forums for the sake of advertising, or just to annoy us. At least that's what I've gathered from how it is used in context.

This may belong in another forum...

ah...thanks.

Actually, it may even be thought of as two separate ideas ingeniously merged into one meaning. "A troll" is a nasty little mythical creature whose sole purpose is to be nasty. "To troll" can be used in a sentence such as "I went out to the lake to troll for crappie today", which means I went out and fished by using a quiet electric trolling motor to putter around in my boat.
So, "a troll" is a nasty little creature who "trolls" around looking for people to take his bait of mischevious intent. :evil:

Kapt.Krunch
(Not a troll...but still mischevious) :roll:

http://en.wikipedia.org/wiki/Internet_troll

http://www.encyclopediadramatica.com/index.php/Jp22

(Dead Link Removed)