Okay my turn...:
The differential has three jobs:
- To aim the engine power at the wheels
- 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
- 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.)
A "locker" allows user-selectable lock-up of the mechanisms allow differentiating wheel-speeds.