An universal joint is named after the style of coupler used to create the joint. Shafts involved are fitting with a "U" style of coupler that is connected by an "X" member (U joint) with bearing on each of the four ends. These bearings allow the "U" joint to operate under extreme load.
Grease is used to lubricate the joint they can either be installed by a pressurized grease gun or by the manufacturer when the universal joint is assembled. This joint allows the shafts to be connected while axis's pivots change, for example: when suspension operation occurs. A typical "U" joint has grease seals on each bearing cap so a boot is not needed like a CV joint. A "U" joint is much like the CV joint in that it doesn't require service unless fitted with a zirk fitting meant to allow grease to be inserted.
There are other types of flexible couplings such as a Thomas or Oldham joint.
These joints use a rubber fiber flex material and are mainly used for low power applications. Sometimes, when replacing a "U" joint you may need to replace the axle assembly depending on the design of the Car. Some manufactures allow the joint to be replaced separately; you will need to check with a part supplier to inform you on which type your car has.
When a "U" joint fails it can cause a rapid clicking or squeaking noise that occurs when the car is being driven and is usually more audible at slower speeds. Also, a "U" joint can cause a fine rust colored powder in and around the "U" joint caps when the joint has failed. This dust is the result of metal contacting metal with no lubricant under extreme pressure.