Moonbean is correct. Most commonly "troll" is used as a verb these days. If you're trolling someone, you might believe in what you're saying, or you might be pretending to believe in it, because the point of trolling is to make someone angry or upset, not to change someone's mind or teach them something. It almost always refers to an online interaction, but as the online world and real life merge more and more, I believe in the future it will become normal to refer to someone as "trolling" even because of their argumentative and reactionary day-to-day conversations in the future.
OH! I forgot, there is another way to use the word, it's slang. You might be "trolling" for something or someone if you're out trying to capture it, and it has sleazy implications. I've heard the phrase "trolling for girls," and I believe this comes from the phrase "trolling for fish," trolling is a fishing method where the fishing lines are moved through the water to increase the chance of catching a fish either by the bait catching their attention or possibly even hooking them unexpectedly as they swim by. Terrible, I know, I don't eat or kill fish. Anyway, I guess this is where the online term comes from, since a "troll" is laying bait and putting out a bunch of lines and hooks, just aimless online looking for attention and outrage from anyone.