Joints are used to constrain the motion of the rigid bodies between each other.
A single joint represents a constraint between two rigid bodies. When the motion of the first body of the joint is known, the relative motion of the second body has at most six degrees of freedom (three for the translation and three for the rotation).
The different joints can reduce the number of degrees of freedom between two rigid bodies. Some joints have limits to control the range of motion and some joints have motors to automatically move the bodies of the joint at a given speed.