A differential enables a pair of wheels powered by the engine to spin at different speeds, but that means differentials have to function differently in vehicles with different drivetrains:
In a rear-wheel-drive (RWD) car, the differential is located between the rear wheels. That’s why it’s called a rear differential.
In a front-wheel-drive (FWD) car, the differential is located between the front two wheels and is also known as a transaxle because it does the job of a transmission and drive axle, too.
Four-wheel-drive (4WD) and all-wheel-drive (AWD) vehicles have both front and rear differentials.