According to Wikipedia 'A troll is someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking other users into a desired emotional response'.
E.g All the comments that lead to this....
However there is a new species of people called Cannibal Trolls. They are people who lure trolls into their trap and make them waste their time baiting who they think is an innocent victim when really they are thriving off the attention. The best example of this is Angry Ginger kid. He is very measured with his response and knows exactly what he is doing when he is provoking this response.
I think we have seen this recently on Mumbrella with Jothy Hughes with his baiting of all the usual Mumbrella commentors with his 'Guaranteed Model of Publicity'.