Generally speaking, you have three basic types of differential, open, limited slip and locked.

With an "open" diff, if one tire loses traction and spins, the engine will send all of it's power to the wheel that sliping and none to the tire that actually has enough traction to propel the car. So, if you have RWD and your left-rear is sitting in oil, you can hit the gas hard and spin the left-rear without any forward movement.

A limited slip design, either through gears or clutches, redistributes some of the power to the non-slipping wheel. Accordingly, as in the example above, the right-rear tire will receive power and the car will move in the direction of tire rotation.

Properly designed and properly tuned, limited slip in 1, 1.5 or 2 way design allows you to apply more power more effectively when you drive through a corner.

With a locked differential, both the inside and outside wheels will move at the same speed.