Definitions from urbandictionary.com:
One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument.
A member of an internet forum who continually harangues and harasses others. Someone with nothing worthwhile to add to a certain conversation, but rather continually threadjacks or changes the subject, as well as thinks every member of the forum is talking about them and only them. Trolls often go by multiple names to circumvent getting banned.