What is "trolling"? If someone calls another a troll, what's it mean?

It stands for a hundred-and-one things, but...

It's a verb that belongs to an 'Internet troll'. When you go about trolling, you basically cause mayhem, post S.P.A.M [stupid-pointless-annoying-messages], say rather rude things to people, and do stupid things.

If someone calls someone else a troll, they don't necessarily need to be an actual troll, they just have to be doing something that severely irritates the other person at the moment. But, they could also be a troll at the same time.

"Trolling" is when you just interrupt a conversation, insulting persons or things that are completely irrelevant to the subject. Trolls do this intentionally, and try to cause MAXIMUM annoyance or destruction.

http://en.wikipedia.org/wiki/Troll