http://en.wikipedia.org/wiki/Internet_troll
and
Usage of the word is various. In some usage, a troll is a communication such as a usenet post, and a troller is the author of such communication. In media where communication isn’t discrete, a troll is the creator of such communication.
A troll is deliberately crafted to provoke others with the intention of wasting their time and energy. A troll is a time thief. To troll is to steal from people. That is what makes trolling heinous.
Trolls can be identified by their disengagement from a conversation or argument. They do not believe what they say, but merely say it for effect.
Trolls are motivated by a desire for attention by people and can’t or won’t acquire it in a productive manner.
Someone may be insufferable, infuriating, fanatical, and an ignorant idiot to boot without being a troll.
Also note that a troll isn’t necessarily insulting, snide, or even impolite. Only the crudest, most obvious, forms of trolling can be identified so easily.
If you find yourself patiently explaining, at length and in great detail, some obscure point to someone who isn’t even being polite to you, then you are probably being trolled.
from http://c2.com/cgi-bin/wiki?TrollDefinition