From the Wikipedia: "In Internet terminology, a troll is a person who posts inflammatory messages on the internet, such as on online discussion forums, to disrupt the discussion or to upset its participants. The word, or its derivative, "trolling", is also used to describe such messages or the act of posting them."
Further, "Various users have different motivations for trolling. A common factor to most of them is the desire to draw attention to the troll. Inflammatory, sarcastic, disruptive or humorous content is posted, meant to draw other users into engaging the troll in a fruitless confrontation. The more attention the troll's activities draw from users, the more persistent the troll's behavior in the forum. This gives rise to the often repeated protocol in internet culture: 'Do not feed the trolls.'"