A person whose sole purpose in life is to seek out people to argue with on the internet over extremely trivial issues. Such arguments can happen on blogs, Facebook, Myspace and a host of others.
The best thing you can do to fight an internet troll is to not answer..or report them.

An Internet troll, or simply troll in Internet slang, is someone who posts controversial, inflammatory, irrelevant or off-topic messages in an online community, such as an online discussion forum or chat room, with the primary intent of provoking other users into an emotional response or to generally disrupt normal on-topic discussion.

A mythological internet being that lives under an internet bridge. Loves to hunt for innocent netizens.

Common tactics: antagonizing other netizens by posting racist or offensive comments
Weakness: being outwitted or unable to antagonize others

A person who posts remarks or comments onto internet forums or message boards in an attempt to get someone to comment negatively to it and to redirect attention onto himself. Usually, these remarks are controversial, stupid, off-topic, inflaming, illogical, or childish. Sometimes, the comments are enough to enrage the people in the forum to want to respond back with their own negative remarks which starts a flame war and changes the topic and attention of the discussion. Since, internet trolls are attention whores, this is exactly what they want since they probably don't get enough attention in real life and needs someone else to acknowledge their self-worth and existence. Nowadays, they are commonly found infesting internet forums and websites such as the GameFAQS Message Boards, youtube (most notably in the comments section), moviecodec, rpgcodex and many others. It is best to ignore them unless you want flame wars to engulf the forum.

Freaking assholes who try to piss you off on places like YouTube vids and such, they're really annoying and will keep you going back and forth for HOURS!

A person who feeds off the anger of others as he enrages them over the internet.

Someone who takes pleasure in causing drama by arguing with people without wanting or accepting anyone's response, also known as a "Justin"