Much of the action in The Outsiders is driven by class conflict. Fourteen-year-old narrator Ponyboy's gang, the Greasers, hail from the economically struggling East Side, while the rival gang, the Socials, come from the wealthy West Side. And, boy, these two groups are locked in a battle with no winners. As author S.E. Hinton tells us that the "Soc vs. Grease conflict" was inspired by similar rivalries in her own high school (source). The Outsiders is concerned with internal conflict and with its characters' inner lives, but most of the action and conflict is framed by interactions within and between groups.
The Outsiders makes an argument for a society in which wealth is more evenly distributed among society's members.
The Outsiders argues that violence and criminal activity aren't isolated to people in the lower economic classes, but that people in the lower economic classes are punished more often and more severely for any crimes they commit.