The shoulder is a very flexible joint that is made up of several tendons, ligaments, and muscles that all work together. Should pain can result from injuries, general wear and tear, and a number of inflammatory conditions.
The top of the upper arm fits in to the glenoid, a round socket in the shoulder blade. A set of muscles and tendons called the rotator cuff keep the shoulder joint in place and provide mobility and stability.
Source: Medical News Today