East Egg and West Egg are both enormously wealthy suburbs of New York City, located on Long Island where they face the ocean. East Egg is the home of those people who enjoy the highest social prestige, as well as their money. Their fortunes have been inherited and their roots run deep in American society. Theirs is "old money." The East Eggers place great value on tradition, family background, social convention, and manners, and they look with contempt upon others who were not born to their kind of wealth. The Buchanans live in East Egg. Tom and Daisy are example of the old money and social snobbery of East Egg.
Those who live in West Egg, like Gatsby, are also very wealthy, but they are the social newcomers who have made their money through commerce (legal or otherwise). They lack the sense of entitlement found among the East Eggers, and they are not "refined" or "polished" in their manners. Gatsby represents this social class. He owns a mansion and dresses well, but he lacks the background of an old and well established family. He is uneducated. He has a great deal of money, but he displays it very conspicuously--a sign of terrible taste to someone like Tom Buchanan.
By developing the social differences between East Egg and West Egg, Fitzgerald develops one the novel's themes. No matter how wealthy Gatsby might become, he would never belong to the Buchanans' upper social class because he was not born into it. He would always be an outsider.