Your shoulder joint is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). Your shoulders are the most movable joints in your body. They can also be unstable because the ball of the upper arm is larger than the shoulder socket that holds it. To remain in a stable or normal position, the shoulder must be anchored by muscles, tendons and ligaments. Because the shoulder can be unstable, it is the site of many common problems. They include sprains, strains, dislocations, separations, tendinitis, bursitis, torn rotator cuffs, frozen shoulder, fractures and arthritis.
Usually shoulder problems are treated with RICE. This stands for Rest, Ice, Compression and Elevation. Other treatments include exercise, medicines to reduce pain and swelling, and surgery if other treatments don't work.
NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases