Constant velocity joints, more commonly called CV joints, are a more sophisticated version of the universal joint. U joints can transmit rotational motion around an angle (such as a driveshaft), but the driven shaft's rotational speed will be uneven due to the design of a universal joint. A CV joint, on the other hand, is designed so that the rotational speed of the driven shaft does not speed up and slow down, but instead is an even speed to match the driving shaft. CV joints are commonly used in driveshafts for front wheel drive and all wheel drive autos, and in steering columns.
There are a few different kinds of CV joints, including Rzeppa joints (pictured top left on this page), tripod joints (used in car driveshafts), Double Cardan joints (used in steering columns and 4 wheel drive driveshafts), and Thompson couplings or Thompson Constant Velocity Joints.