A good troll is one who trolls trollers. Basically, when a troll trolls, another troll defends the one who was being trolled in the first place. A good troll purposely starts arguments with bad trolls. You will notice that a lot of history on that Good Troll's computer is arguments defending someone.
YouTube example:

Bad Troll: This video sucks. Get off youtube, faggot.

Good Troll: You know what really sucks? When a lazy ass guy who doesn't bother to get up and get a life posts not-needed comments because he has nothing else to do. That sucks.
by Buffy fan March 26, 2011
Get the Good Troll mug.