When a car makes a left turn, the interior-facing wheels (on the left side) naturally turn slower than the exterior-facing wheels (on the right side) because the exterior-facing wheels have to travel farther to get the car around the corner. For this reason, the wheels of a car need to be able to revolve independently and at varying speeds to prevent slipping, and to keep the car moving with power around corners.
Whether or not your car has one or two differentials depends on how the wheels of your vehicle are powered. If you have a rear-wheel drive vehicle your car uses engine power to move only the rear wheels, so you will only have one differential mounted on the rear axle. If you have an all-wheel or four-wheel drive vehicle your car is built in a way that allows the engine to send power to all four wheels, so you will have both a front differential and rear differential. If you have a front-wheel drive vehicle, instead of a differential you will likely have a transaxle that functions as both the transmission and the differential. Rear wheel drive vehicles will have a differential in the rear end of the car.