The shoulder joint is a ball-and-socket joint and can rotate in many directions. This flexibility makes it the most mobile joint in the body but also makes it an easy joint to dislocate.
A shoulder dislocation occurs when the ball-head of the upper arm bone (the Humerus) shifts out of the glenoid socket of the shoulder blade either due to a violent blow or extreme rotation of the shoulder.
The dislocation could be partial (subluxation) or complete, both of which cause pain and instability in the shoulder.