The Shoulder Joint comprises a ball and socket. The unique characteristic of this ball and socket joint (unlike the stable hip joint) is a large ball sitting on a small socket leaving a large portion of this ball uncovered. This is quite similar to a golf ball sitting on a tee.
Above the Shoulder Joint (Glenohumeral Joint) traverse a set of muscles known as the Rotator Cuff muscles which attach to the sides of the ball at the Greater and Lesser Tuberosities. Just above these muscles is a bone which one can feel as the tip of the shoulder - The Acromion. A soft jelly like cushion (the Subacromial Bursa) between the rotator cuff tendons and overlying acromion bone prevents undue rubbing of the tendons against bone. The space between the acromion above and the humeral head below is known as the Subacromial Space.
In front of the shoulder joint traverses a tendon known as the biceps which travels down to the elbow.
The shoulder joint is covered all around by the capsule which is quite similar to a polythene bag covering a bone from all sides. This renders the joint watertight.
The rotator cuff muscles are attached to the Scapula (shoulder blade) at the back. There are some other big muscles at the back attached to the shoulder blade.