There are multiple types of trolls, but the two most common are:
1. Racist/inflammatory/homophobic troll: This person posts what they know a priori to be inflammatory comments, with the intent of getting people all riled up. This tends to be fairly obvious.
2. Naive-“stupid” troll: This type of troll is a bit more sophisticated and insidious. They either post questions, or comments that invoke the “What are you STUPID?” remarks, and keep on going with the baiting until they are satisfied or the irritated user(s) give up.
I would post examples, but I don’t want to embarrass PnL anyone. ;-)