You can find the answer to this question on page 33, the beginning of Chapter 3.
Ponyboy is speculating about what separates the Socs and Greasers, because he realizes that Cherry and Marcia are a lot like Two-Bit and himself when the movie ends and the boys offer the girls a lift home in Two-Bit's car.
Ponyboy thinks it's just money that separates them, but Cherry says money's part of it, but there's more separating them than money alone: "'It's not just money. Part of it is, but not all. You greasers have a different set of values. You're more emotional. We're sophisticated—cool to the point of not feeling anything.'"
Cherry goes on to say that she is not always honest with her friends: "'I'll catch myself talking to a girl-friend, and realize I don't mean half of what I'm saying.'" Then she tells Ponyboy that Socs are always on the go, trying to achieve more, and never satisfied with what they have: "'It seems like we're always searching for something to satisfy us, and never finding it.'"
The conversation finishes when the two of them realize that they watch the same sunset from different sides of town: "Maybe the two different worlds we lived in weren't so different. We saw the same sunset" (page 35).