Posting rude and distasteful comments, sometimes for the purpose of a reaction and other times not. Either way it is malicious and whether it is intended or not can have serious consequences. Those who troll will laugh at that, but trolling fosters an environment that leads to more serious abuse also known as cyberbullying where lines are crossed which should never, ever be--like encouraging someone who is suicidal to commit suicide. This is bad humor, and while in your virtual reality it may be funny, in actual reality it's not. It's time to grow up.
See YouTube.com for endless examples of trolling.