A U-joint (the "U" stands for "Universal"), which is also called a "Cardan" joint after the guy who invented it, is a type of flexible coupling typically used on both ends of the driveshafts in rear-wheel and four-wheel drive vehicles. Each U-joint consists of a four-legged center cross with needle bearing cups on the ends of each leg of the cross. The bearing cups on one pair of legs are mounted to the driveshaft. The other pair of cups are held in place by a pair of U-bolts attached to a yoke that mates to either the transmission or differential. The bearing cups allow the joint to swivel and bend as the driveshaft follows the motions of the differential and axle as the suspension bounces up and down.
Most original equipment U-joints on newer vehicles are "sealed" and do not require periodic greasing. But many replacement U-joints as well as the U-joints on older vehicles do have a grease fitting which allows the joint to be lubed periodically.
A constant velocity (CV) joint does essentially the same thing as a U-joint, only better. There are two basic types: "ball-and groove" CV joints (called "Rzeppa" joints after the guy who invented them), and "tripod" CV joints.
Rzeppa CV joints, which are used as the outer joints on most front-wheel drive cars and minivans, consist of a cup-shaped outer housing, a center race and cage assembly. Machined into the outer housing and center race are six grooves that hold six steel balls. The balls are held in position by windows or slots cut into the cage assembly. The joint is designed so that when it bends, the balls are always positioned at the midway point inside the joint. This eliminates the cyclic variations in speed that a U-joint experiences when it operates at more than a few degrees off-center.
A variation on the Rzeppa CV joint is the "cross-groove" CV joint. It also has six balls between an inner race and outer housing. But this type of joint is designed to move or plunge in and out to compensate for changes in driveshaft length that occur as the suspension moves up and down. This type of joint is used as the inboard CV joint on many European and Japanese front-wheel drive cars.
The tripod style of CV joint consists of a three-legged cross or trunnion with roller bearings on the end of each leg. The trunnion is attached to the driveshaft, and the roller bearings run in machined grooves or channels in an outer "tulip" housing. This type of joint is also designed to plunge in and out, and is used as the inner CV joint on most domestic front-wheel drive vehicles. There are also some Japanese and European front-wheel drive cars that use a tripod-style joint as the outer joint.
All CV joints are enclosed by a rubber or hard plastic boot. The boot keeps grease in and contaminants out. CV joints do not require periodic maintenance or greasing, and are engineered to last 100,000 miles or more.
All front-wheel drive cars and minivans have four CV joints: one inner joint and one outer joint on each of the vehicle's two driveshafts (which are also called "halfshafts"). CV joints are also used on the driveshafts of some rear-wheel and four-wheel drive vehicles, too.