A social outcast is anyone that doesn't want to be a part of society, or just doesn't fit in. They tend to be pushed out of the many 'social groups' that exist all around us. They may either like being one, or hate it.
When a person enjoys being a social outcast, they feel that they are either superior to others, or they just don't want to be around the majority of people in this world.
If the person hates it, then they tend to become depressed or if it's extreme, suicidal. Then they try their best to fit in, only to try to hard and get even more segregated from others.
A prime example of what happens to social outcasts is the Columbine Shooting. They were picked on (as far as the media says) by many people. Nobody seemed to like them. And if they stood up, everyone would bring them down. That's why all that mental frustration built up and they decided to do what they did.
This all really depends on how anti-social you like to be...