Old money and new money.
They are locations for the major events in the story. Nick and Gatsby live in West Egg, and Daisy and Tom live in East Egg.
East egg is old money, people who grew up with money, and West egg is new money, people who just got their money
I am hoping you are referring to the Great Gatsby. The are is known as the Valley of Ashes, which is the industrial part of town.
West Egg is modeled after Great Neck, NY and was considered the "new money" area. East Egg represents "old money" and a more refined culture.