Within the drive train, power is transmitted as rotation, usually along driveshafts. For various reasons relating to suspension and dynamic loads on the drive train, one end of a drive shaft will move out of line with the other end. Universal Joints and Constant Velocity Joints (CV Joint) are components that allow a drive shaft to pivot, while still turning, allowing the drive shaft to essentially flex. A universal joint is an arrangement of 2 yokes and a "spider" when assembled, the joint can flex but any twist placed on one side will be transmitted to the other side. Because of its assembly, a universal joint does not transmit torque smoothly, and must be used with another universal joint to even out.
A constant velocity joint uses a center member, an arrangement of ball bearings and an outer cage. A CV Joint serves the same purpose as a universal joint, but it has the ability to smoothly transmit rotation, throughout its range.