The shoulder joint is considered a ‘ball and socket’ joint, however, the ‘socket’ (the glenoid fossa of the scapula) is quite shallow and small, covering at most only a third of the ‘ball’ (the head of the humerus). The socket is deepened by the glenoid labrum . The glenoid labrum is a firm, white structure that form a ring around the glenoid cavity (the cup of the ball and socket shoulder joint). 

The labrum is made of fibrous cartilage tissue, which is similar to the meniscus in the knee.
The labrum is triangular in shape in cross-section and thus deepens the socket of the shoulder. It also provides a ‘suction-cup’ effect to the joint. These two functions means that it is extremely important in providing a form of stability to the shoulder joint.