Basically, an individual. In the school social classes of today, a misfit doesn't fit into any one quite right, not even the outcasts, but may have qualities of each one. True misfits usually do not believe they are emo or goth, and they are usually introverts. Misfits tend to follow their own beliefs, and are usually persecuted for it. Misfits tend to be hated for no reason, have few good friends, and are usually intelligent and mature, and sometimes sort of insane and depressed. Misfits tend not to care about their bad social lives, but some do.
That quiet, depressed girl over there is a misfit.