to me a troll is someone who posts stuff just to get a rise/reaction or get under people's skin...
their posts are often reactive so if you say the sky is blue they'll come in with "what do you mean , the sky is black...."...to me that's often trolling..
To me, trolling is pretending you believe a certain thing that's provocative/outrageous in hopes of soliciting angry responses from people who are under the misguided view that you actually believe said thing. Trolling doesn't "work" if people are not convinced that the troll ACTUALLY believes what they're saying, otherwise they won't get the requisite level of "upset" to get into an argument over it. I don't want to get into too META of a discussion but the thing that's weird about this community is many don't seem to see the difference between intended satirical material in the vein of TheOnion.com and "trolling."
For example, you might not like The Onion. You might not think its funny. You might think its satire is too hamfisted or its points too obvious. Those are legitimate complaints. But to say The Onion is "trolling," for example, is absurd. No one is "trolled" by The Onion. Nobody reads "Voters Shocked Christie Botched Such An Easy Political Cover-Up" and actually thinks that's true. But if you post that same thing here, there's 20% of this board who is going to argue tooth and nail, "THAT ISN'T TRUE! AND LET ME SHOW YOU EMPIRICAL EVIDENCE TO PROVE VOTERS AREN'T ACTUALLY UPSET ABOUT THAT!"