The definition of a troll varies. Usually its an anonymous user going into the comment section, and looking to stir up trouble by saying mean stuff, purposely being dumb or making a random, stupid comment, just to rile people up. The saying 'Don't feed the troll' means to simply ignore these trouble seekers, because they probably are looking for the attention of others. I hope I helped.
A person posting with the primary goal of causes a person to lose there cool. It gets thrown around loose on gag. If someone gives the blunt truth, they are often called trolls by the person who is overly sensitive.