An Internet troll, or simply troll in Internet slang, is someone who intentionally posts controversial or contrary messages in an on-line community such as an on-line discussion forum with the intention of baiting users into an argumentative response.
Trolls can be existing members of a community that rarely post and often contribute no useful information to the thread, but instead make argumentative posts in an attempt to discredit another person, concentrating almost exclusively on facts irrelevant to the point of the conversation, with the intent of provoking a reaction from others.
A troll's main goal is usually to arouse anger and frustration among the message board's other participants, and will write whatever it takes to achieve this end. One popular trolling strategy is the practice of Winning by Losing. While the victim is trying to put forward solid and convincing facts to prove his position, the troll's only goal is to infuriate its prey. The troll takes (what it knows to be) a badly flawed, wholly illogical argument, and then vigorously defends it while mocking and insulting its prey. The troll looks like a complete fool, but this is all part of the plan. The victim becomes noticeably angry by trying to repeatedly explain the flaws of the troll's argument. Provoking this anger was the troll's one and only goal from the very beginning.
The general element, that determines whether a malicious user is a troll or not, is the level of indignant emotions present in the person, coupled with the person's history with the forum or group. An indignant user who has had a previous normal relationship with the group is not a troll, even if the user uses methods of attack that are characteristic of a troll attack.
What to do if you find a troll
By calling someone a troll, you outright label them as having unkind intentions. Sometimes what you think is a troll is merely a dissenting opinion. There is no requirement that everyone must share an opinion, however combating a troll is a loosing proposition.
The best thing to do is ignore them. A moderator will take care of it.