The Socs are the rich kids, and the Greasers are the poor kids. In order for one to become a member of the other group, he would have to change social class and basically gain or lose money.
The distinction between the Socs and Greasers is social class. Ponyboy explains that “organized gangs are rarities-there are just small bunches of friends who stick together, and the warfare is between the social classes” (p. 12). They fight each other because they are gangs, but the gangs are based on neighborhood and class distinctions. Ponyboy notes that, “I'm not saying that either Socs or greasers are better; that's just the way things are” (p. 4).
The Socs are described as “the Socials, the jet set, the West side rich kids” (p. 3). The Greasers are from the East side, a less desirable neighborhood.
Ponyboy says that the Greasers are “poorer than the Socs and the middle class” and wilder (p. 4). He describes the Greasers as “almost like hoods” because they “steal things and drive old souped-up cars and hold up gas stations and have a gang fight once in a while” (p. 4). Physically, they “just like we wear our hair long and dress in blue jeans and T-shirts, or leave our shirttails out and wear leather jackets and tennis shoes or boots” (p. 4).
The Socs, on the other hand, “jump greasers and wreck houses and throw beer blasts for kicks, and get editorials in the paper for being a public disgrace one day and an asset to society the next” (p. 4). Part of the bitterness the Greasers feel is that they cannot get ahead, because “you can't win against them no matter how hard you try, because they've got all the breaks and even whipping them isn't going to change that fact” (p. 12).
In the end, Ponyboy realizes through his association with Cherry that the Socs and Greasers are artificial barriers. He says
Maybe the two different worlds we lived in weren't so different. We saw the same sunset. (p. 42)
Probably the Greasers would be like the Socs, spoiled. And the Socs would be like the Greasers.