The shoulder joint is the junction of three bones: the upper arm (humerus), the shoulder blade (scapula), and the collarbone (clavicle). It is classified as a ball-and-socket joint but is extremely shallow. The shallowness of the joint allows for the shoulder's incredible range of motion. However, due to this increased range of motion, the shoulder is an inherently less stable joint and injury is a common occurrence.

Shoulder dislocation occurs at the ball-and-joint where the humerus meets the scapula, the glenohumeral joint. This type of injury is characterized by the humerus becoming detached from the scapula. Anterior dislocations, where the humerus shifts to the front of the scapula, are most common. Posterior dislocations are less common.

Further Reading:

American Academy of Orthopaedic Surgeons: Dislocated Shoulder