A person who posts rude, off topic, or other twisted comments in the name of angering people. There are 2 types of internet trolls though. Troll is a shortened version of the term Troller, that means the same thing.
Bad troll: An idiot who can't win in arguments in real life, so he goes on the internet and posts random bullshit to make himself feel better about being a moron. You can usually find these on chatrooms like AIM and comment boards like on Youtube. The also troll other comment boards like aol news (espicially kidnapping cases since they try to look like they did it to look beast)
Good Troll: Rare, but still in large numbers. These are similar to bad trolls but do it for the good of getting idiots off the web, such as trolling someone on Youtube who posts everything their dog does or someone acting tough with a keyboard. Think of it as the Asassain's Creed story, The good trolls are the Asassains and the morons and bad trolls are the Templars.
Example: Good troll finds bad troll and they fight.
Bad Troll(on a kickass mw2 knife kill) OMG NURD GO PL3H BFBC2 IS W3H BETTER TAHN THIS YOU EAT DICK N00BER!!!!
Good troll: Bad Company 2 hasn't come out yet :D.
Bad troll: STFU your a retard i'm going to kill your family.
Good Troll: Good luck E-thug....
Example 2: Good Troll finding someone taped there dog pooping and posted it on YT.
Good Troll: A little privacy for Muffy here would be cool?
Creator: OMG REPORTED LOL U FAIL!
Good troll: ..... youtube doesn't have a report function nor do they give a crap about you and me.