A vehicle's differential is the part that lets the rear tires move at different speeds, enabling your car to turn efficiently.
It is generally located between the two rear wheels.
The inside wheels, or wheels closest to the turn, move around a corner at a lower speed and smaller distance than the outer wheels. The outer wheels keep up with the inside wheels by traveling at a greater speed over this distance. This Learn Engineering YouTube video explains more about how the differential operates.
It is important to change your oil and perform other maintenance on the rear differential to keep it running smoothly. Ideally, the oil should be changed after every 30,000 miles.
The Carbone Automotive Group in Yorkville, New York has experienced service technicians who will be able to answer your questions, including those relating your differential.