A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it combines two inputs to create an output that is the sum, difference, or average, of the inputs.
In automobiles and other wheeled vehicles, the differential allows each of the driving roadwheels to rotate at different speeds.
The differential has three jobs:
1. To aim the engine power at the wheels
2. To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
3. To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)