The shoulder is a ball-and-socket joint that allows the arm to rotate with a circular movement with the ability to hinge out, up and away from the body.
The primary bones in the shoulder joint are the humerus and the scapula. The humeral head articulates against the socket of the scapula, called glenoid, during movement. The surface of the bones is coated with a smooth tissue called articular cartilage. The cartilage, together with a substance called synovial fluid, prevents the bones from rubbing against each other and causing pain.

The rotator cuff is a group of four muscles and the corresponding tendons that surround the shoulder joint and provide stability to it.