The shoulder is made up of three bones: upper arm bone (humerus), shoulder blade (scapula), and collarbone (clavicle). It is a ball-and-socket joint, meaning the ball, or head, of your upper arm bone fits into a shallow socket in your shoulder blade. This socket is called the glenoid.
The surfaces of the bones where they touch are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. A thin, smooth tissue called synovial membrane covers all remaining surfaces inside the shoulder joint. In a healthy shoulder, this membrane makes a small amount of fluid that lubricates the cartilage and eliminates almost any friction in your shoulder.
Together, with the muscles and tendons that surround the shoulder, all of these structures allow the shoulder to rotate through a greater range of motion than any other joint in the body.
Content courtesy of American Academy of Orthopaedic Surgeons.