What's a black sheep?

This seems thrown around a lot to me, usually as a claim by someone who wants to be different. Seems common enough to me, anyway. I’m a rebel, an extremist, a black sheep…I hear people say this often about themselves.
But what is a black sheep, exactly? I don’t think I really know. All I get from people are that they deny or dismiss a thing, concept, fashion, whatever. Not listening to MTV doesn’t make you a black sheep, does it? I’m not a black sheep cuz I’m a Goth right? (One Goth among a million Goths, and so forth.) So what is one? How is one a black sheep, how does one go against the current, how does one do everything differently than everyone else to be a black sheep? Social norms? Does that have anything to do with it? Couldn’t I be a black sheep if I lived on my own in some shack in a forest?
Or is it a mindset that’s uncommon? Is it good or bad? Are you born a black sheep, is it something in the brain, or does it come as a form of revelation from experience? What social factors might be related to being a black sheep? What about psychological? Are you a black sheep? Am I? What is a black sheep? That is, beyond the general definition.