A universal joint is a mechanical device that allows one or more rotating shafts to be linked together, allowing the transmission of torque and/or rotary motion. It also allows for transmission of power between two points that are not in line with each other.
Universal joints come in a wide variety of shapes, sizes and configurations to accommodate the infinite amount of applications they can go into. You should always consult a professional before selecting a universal joint.
Universal joints allow drive shafts to move up and down with the suspension while the shaft is moving so power can be transmitted when the drive shaft isn’t in a straight line between the transmission and drive wheels.
Rear-wheel-drive vehicles have universal joints (or U-joints) at both ends of the drive shaft.