More specifically, trolling is a way of fishing. What you do is bait a hook, and dangle it off the back of a boat then drive the boat around waiting for a fish to bite. It's been adapted for use on bulletin boards like this one. It's been taken so far as to say one who trolls is a troll. Bad English for sure, but that's what has evolved on the web. So a troll is someone who posts a message that is inflammatory -- that he knows will cause a stir -- and waits for someone to bite on it. It more or less always causes fights.