http://en.wikipedia.org/wiki/Internet_troll
On the Internet, the term troll is a slang term used to describe a number of things, traditionally:
A post (on a newsgroup, or other forum) thought to be intended to incite controversy or conflict or cause annoyance or offense.
A person who posts these.
The term troll has negative connotations, and is often applied as an insult. An individual posting honestly-held but controversial opinions is sometimes mis-labeled as a troll. By contrast, a troll's primary incentive is generally considered to be the incitement of conflict.
A troll's reactions to being identified as a troll often gives the game away. A person unjustly accused of being a troll may be hurt and express indignation. A troll is more likely to react with verbal abuse, raising the stakes with inflammatory remarks maligning the motivation of the accuser.
Trolling is often described as an online version of the breaching experiment, where social boundaries and rules of etiquette are broken. Self-proclaimed trolls often style themselves as Devil's Advocates or gadflies, challenging the dominant discourse and assumptions of the forum they are 'trolling' in an attempt to subvert and introduce different ways of thinking. Detractors claim that true "Devil's Advocates" generally identify themselves as such for the sake of etiquette whereas trolls do not feel so bound.
Trolls are sometimes caricatured as socially-inept. This is often due to fundamental attribution error, as it is difficult to know the real traits of an individual solely from their online discourse. Indeed, since intentional trolls are alleged to knowingly flout social boundaries, it is difficult to typecast them as socially inept since they have arguably proven adept at their goal of inciting conflict.