The differential is a device that splits power between 2 wheels. As a car drives straight, its wheels will travel the same distance and rotate at the same speed. When a car turns, the outside wheels will follow a longer path around the curve. If the inside and outside wheels were connected by a mechanically solid axle, they would force each other to rotate at the same speed. The speed difference between the wheels on the shorter, inside path and the wheels on the longer, outside path would result in tire wear, poor handling, and if the traction of the tires were too great, the axle itself could be damaged.
Automobiles can have several types of differentials; Open, Limited-Slip, and Locking. An open differential lets either wheel spin at any speed traction allows. The majority of cars on the road have this kind of differential. A limited slip differential has either viscous liquid or small clutch packs to allow power to go to the wheel with traction while the other wheel is spinning without traction. This type of differential is mostly seen on sports cars. A locking differential uses a mechanical connection to lock or unlock both wheels. The only usual application for this is off-roading in trucks or SUVs.