In New York City, "Uptown" means the area north of 57th Street around Central Park and especially, Park Avenue. It's associated with the rich and glamorous of the city. Backstreet boy is not a specific place, so it's not a direct contrast, but it just means someone from the rougher part of town off the main streets.

High class. Uptown being where all the rich people live in town. The lyrics in his actually explain the meaning

rich girl , poor guy