Ask Wikipedia. http://www.wikipedia.org/wiki/Internet_troll

"On the Internet, a troll is a person who posts messages that create controversy or an angry response without adding content to the discussion, often intentionally. Though technically different from flaming, which is an unmistakable direct personal attack, trolls often resort to innuendo or misdirection in the pursuit of their objective, which is to create controversy for its own sake, discredit those with whom they disagree, or sabotage discussion by creating an intimidating atmosphere.

Note that this is a highly subjective term, as everyone is affected differently by the nature of the term deemed a "troll".

Originally this term applied to people who were intentionally posting flamebait, by analogy with the fishing technique of trolling: metaphorically, these people were dragging a conversational lure through the group, hoping for a response. The concept of "this person is trolling our newsgroup" became shortened to "this person is a troll", and picked up the association of the monster trolls of folklore. "