There are four types of rear ends used in cars and trucks.
1. Open - The basic rear end used in most vehicles. The tire with the least amount of traction will receive all the torque from the engine. This is the type of rear end that you can have one wheel spinning and the other standing still and will leave you stuck in the mud or snow. You will get one black patch on the pavement if you do a burnout. This rear end provides for the smoothest action when going around corners on pavement. The quick check for an open rear end is to jack the rear wheels off the ground. If you turn one in the forward direction the other will turn in the backward direction.
2. Limited Slip - Often refered to as posi, positraction, suregrip, tru-track and any number of other names that the marketing folks can think up. They use clutches or other mechanical means, such as the Detroit Locker, to ensure that some torque is always applied to both wheels. You will never be in normal driving situation when one wheel is spinning and the other is standing still. You will get two even black patches when doing a burn out (if you don't then the rear end is worn and in need of attention). The units that uses clutches provide for relatively smooth action going around corners, the mechanical ones can be rough as the mechanism grabs and releases. The quick check for a limited slip is to jack the rear end off the ground and turn one wheel in the forward direction and the other will also turn in the forward direction.
3. Locker - (not to be confused with Detriot Locker) is a rear end that can be switched from open to locked (i.e., spool) from inside the cab. These rear ends are typically used by the serious off roaders.
4. Spool - This type rear end has no differential action, that is, both wheels are locked together and always tuurn at the same speed. These rear ends are not practical for street use and are typically seen on drag cars.