The 'true' shoulder joint is the glenohumeral joint - where the top of the arm meets the rest of the body. The shoulder complex includes the collar bone at the front, and shoulder blade to the back. Therefore shoulder pain can cover quite a large area of different structures. The shoulder is extremely mobile, more in fact than any other joint in the body. Whilst this makes it extremely useful, it's also more prone to injury through overuse and trauma.
Tendonitis - Inflammation of a tendon. Occurs with most shoulder injuries, pain tends to be increased on movement.
Bursitis - Inflammation of a bursa (fluid filled sac). Often associated with tendonitis, trauma or rotator cuff tear. Characterised by hot swelling with marked pain on movement.
Rotator Cuff Tear - This is a muscle tear. It usually follows trauma and is very painful.
Impingment Syndrome - Inflammation caused by a reduction of space within the shoulder joint. Often aggravated and indeed caused by frequently using your arms above your head. Associated with Oesteo-arthritic changes to the bones themselves, and frequently accompanied by joint instability.
Instability - When your shoulder is able to move outside its structural limits. If caused by a trauma it is often associated with joint dislocation, and usually very painful. If non-traumatic onset, it can be caused by repetitive and excessive use particularly through throwing sports or swimming, or indeed you can simply be born with it.
Frozen Shoulder (Adhesive Capsulitis) - This occurs when the capsule around the shoulder joint adheres to itself with inflammation. Shoulder movement is greatly restricted and in the acute stage is accompanied by severe pain.