The shoulder is a ball and socket joint and is the most mobile joint in the body, made up by three bones the - the clavicle (the collar bone), the scapula (the shoulder blade) and the humerus (the arm bone). Because it is very mobile the shoulder relies upon a complex arrangement of ligaments and muscles to hold the ball within the socket.
Because the shoulder is so mobile it can be susceptible to the ball jumping out of the socket - or a dislocation. When this happens the ligaments that hold the ball into the socket may be stretched or torn away from the bone and may require repair. Those ligaments are known as the glenohumeral ligaments.
There are two main joints around the shoulder, that connecting the clavicle to the scapular - the ACJ or acromioclavicular joint, and the one connecting the scapular to the humerus - the glenohumeral joint. Both of these joints may be affected by arthritis and become significant sources of pain within the shoulder region.