Drivingfast.com describes in simple terms, a differential, as a device which allows for the differences in wheel speed which naturally occurs when a car turns at a corner. We shall promptly dispatch that definition out the nearest window. This was clearly written by a physicist in complete disregard to the rest of us. How stuff works defines it as a device that splits the engine torque two ways, allowing each output to spin at a different speed. Let us work with this.
Put quite simply, it is a system of gears that allows the wheels of a car to turn at different speeds, thus allowing it to turn smoothly. Let us focus on a rear wheel drive car. Essentially, when a car is moving in a straight line, all the wheels are rotating at the same speed and covering the same distance.
That, however, all changes when the car goes into a corner. The laws of physics dictate that for the car to turn smoothly, the outside wheel is supposed to cover a larger distance than the inside wheel. For that to happen, they need to spin at different speeds.
Now, there is no way those wheels will spin at different speeds if they are locked together. To force a car to navigate a corner with locked wheels will either make the car skid hopelessly, or worse, fall over. If you had a tricycle as a child, you know how hard it was to turn. The back wheels were locked together. If one moved, the other moved as well.
To give you another example, anyone who saw or rode the engine powered carts at Didi’s World when it had just been opened, can testify to how annoyingly tricky they were to turn. You spent most of your time in the tyre barrier because you were trying to defy physics.
The stewards came and pulled you out, but you ended up deep into the barrier at the very next corner much to the amusement of the watching committee. No one wants that with their car, and that is where this clever piece of machinery comes into play.
It is situated between the two wheels, and though all you see is an odd looking bump under the car, there is a set of gears inside that work like the cogs of a clock. The minute hand moves slower than the hour hand, yet they remain in perfect synchrony.