One meaning of troll is of course, the monster who lives under a bridge and eats goats. J But here’s the new meaning, in the context of the Internet. It’s an Internet slang.
A troll is one who deliberately makes provocative comments on virtual message boards with the intention of causing maximum disruption and argument. You’ll find a troll in online forums, chat rooms, blogs and message boards. A troll usually doesn’t have anything substantial to add to a conversation but continually threadjacks or changes the topic of the thread by making disruptive posts. Trolls make stubborn, prejudiced and often insulting remarks and try to cause as much admin havoc as possible, usually resulting in getting banned. Trolls would often operate under various user names to avoid getting banned and would ‘go trolling’ i.e make disruptive posts in more than one message board simultaneously. Disrupting a thread is ‘flaming’ and a troll usually flames thread without staying on topic. In contrast, a ‘flamer’ flames a thread because s/he disagrees with the content of the thread.
Here’s more on it: