According to Ponyboy, there are two kinds of people in the world: greasers and socs. A soc (short for "social") has money, can get away with just about anything, and has an attitude longer than a limousine. A greaser, on the other hand, always lives on the outside and needs to watch his back.

Ponyboy is a greaser, and he's always been proud of it, even willing to rumble against a gang of socs for the sake of his fellow greasers--until one terrible night when his friend Johnny kills a soc. The murder gets under Ponyboy's skin, causing his world to crumble and teaching him that pain feels the same whether a soc or a greaser.

(source)