The rear end of rear wheel drive vehicles and four wheel drive vehicles has an assembly called a differential. The differential is designed to allow the power from the engine to be transferred through the rear wheels. The driveshaft goes straight back into the differential and the rear wheel axles come in from the sides. The housing is filled with a special gear oil.
The easiest, and most overlooked, thing to do is to be sure to change the rear end fluid at regular intervals. It's usually a relatively inexpensive service that can save you a couple thousand dollars.