The purpose of the differential is to distribute engine energy to the axles so that the axles spin the wheels the right way at the right speed. The differential also allows the wheels to spin at different rates as a vehicle rounds a corner. The inside wheel spins slower than the outside wheel when turning; this "difference" is how the differential gets its name.
The differential contains gears and bearings that are lubricated by oil. Like all oils and fluids in a vehicle, the oil in the differential should be changed every 30,000 miles or more frequently if you use your vehicle to tow or carry heavy loads.
To drain and refill the differential fluid the vehicle is driven to get the differential oil warm. The oil is then drained from the differential and refilled with fresh oil.