To me, trolling is posting material intended to solicit a hostile reaction without advancing the discussion.
I'm not 100% sure about including intent there, and I'm not 100% sure about including "advancing the discussion".
If somebody posts something in good faith that he thinks is germane to the discussion, and it results in a firestorm of angry replies, I'm not sure that's trolling.
But if you're trying to talk about something and I jump into the thread and talk about how much the Jets suck because I think it's going to get you mad and throw everything off topic, that's definitely trolling.
-k