Wikipedia: 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.

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

Trolling mostly maintains its earlier meaning of posting messages specifically in order to elicit an particular response, usually anger or argument.

The noun form, troll, is sometimes used in the more general sense of someone who stirs up controversy, whether or not the controversy itself is their goal. Frequently this is used to discredit one position in an argument. By asserting that one's opponents are trolls, one is asserting that they are only maintaining their position in order to feed the flames, and that their position is actually indefensible.

Common types of troll messages:

off topic messages
inflammatory messages
messages containing an obvious flaw or error

An example of a troll message would be one that denounces a particular religion in a religion newsgroup.

There is some generally-accepted wisdom about dealing with Internet trolls: "Don't feed the trolls, that will only encourage them." That is, do not respond to them, that is the attention they desire.