In computer terminology, it's a person who is deliberately being misleading. It used to mean someone who was old and ugly and purporting to be young and attractive. Now it's more general.
A troll could also be just an old and unattractive man, like one you would see at a pick-up bar.
The traditional meaning of the word is a supernatural being living underground.
But again, in this environment, it's just someone with a false intent. On Yahoo!answers, they're often teenagers with nothing better to do. They might ask a question just to provoke like "Are dolphins really just gay sharks?" Or they might report a false event such as being harassed for saying grace at a restaurant.
Are you a troll?