A troll is simply someone who wants a reaction. It could be someone saying what they truly believe in a hostel forum, or someone making stuff up. Hell, I was probably a troll when I was polite in a few very nasty message threads. I certainly did it just to see the reaction.

My understanding is that a true troll is someone who says what he or she says specifically to fuck with people and, indeed, get a reaction. It seems to me such a person may or may not be saying something she actually believes.

Not really relevant here, but I've noticed people labeling someone else a troll simply because the person is saying something provacative that they disagree with. I've been called a "left wing troll" for defending the health care law in what were pretty mild terms. I wasn't trolling, but it was an easy out for the other guy to just pathologize me in that way--I was "just a troll" who could be therefore dismissed. The word itself became an excuse to bail on an argument the person was (if I may say so) not winning. Again, not really relevant to OP, but there ya go.

Ebert famously called Armond White a troll, which I didn't agree with not because it was mean but because I felt it was inaccurate.  (I think Armond's whole thing he does is far more complicated, and often way more fucked up and mean spirited, than your average everyday troll.)