The shoulder is a pretty complicated ball and socket joint. It consists of the clavicle (collarbone), scapula (shoulder blade) and the humerus (arm). The humerus is attached into the socket by the muscles, tendon and ligaments only.
There is NO bony attachment. The alignment of the ball into the socket is dictated by the rotator cuff muscles!! They are called the SITS muscles.
The SITS muscles are the, Supraspinatus, Infraspinatus, Teres Minor and the Subscapularis. The Subscapularis is the most common for injury and tears.
Below is a short video.