An "Internet troll" or "Forum Troll" is a person who posts outrageous message to bait people to answer. Trolls delight in sowing discord on the forums. A troll is someone who inspires flaming rhetoric, someone who is purposely provoking and pulling people into flaming discussion. Flaming discussions usually end with name calling and a flame war.
A classic troll tries to make us believe that he is a skeptic. He is divisive and argumentative with need-to-be-right attitude, "searching for the truth", flaming discussion, and sometimes insulting people or provoking people to insult him. A troll is usually an expert in reusing the same words of its opponents and in turning it against them.
While he tries to present himself as a skeptic looking for truth ... his messages usually sound as if it is the responsibility of other forum members to provide evidence that what forum is all about is legitimate.
He (and in at least 90% of cases it is he) tries to start arguments and upset people.
See full article at http://ubuntuforums.org/showthread.php?p=1032102