Your shoulder--a ball-and-socket joint--is the most flexible joint in your body. The ball, or head, of the arm bone (humerus) rests against a small, shallow socket (glenoid). Muscles and other soft tissue hold the ball in the socket and allow you to move your arm up and down, to the side, across your body, and behind your back.
When your shoulder joint is healthy, the ball glides smoothly in the socket. That's because the ends of the bone are cushioned by a smooth covering (cartilage), and tissue (synovium) lines the joint. The muscles and tendons of the rotator cuff hold the head of the arm bone firmly in the socket and give you strength and flexibility.