The shoulder joint is made of 3 bones:

1) The Scapula (shoulder blade) – it has a shallw cavity(glenoid)

2) The Clavicle(collar bone).

3) The Humerus (upper arm bone)-its end is rounded and fits into the glenoid.

A lubricated sac of tissue called bursa between the acromion and the rotator cuff also facilitates movement.Since the shoulder is a non weight bearing joint wear and tear is less common than hip/knee Osteoarthritis/rheumatoid arthritis/fracture/repeated dislocation can damage the joint tissues and result in a stuff and painful joint.

AIt is by cartilage and allows smooth movement.
The tendons of surrounding muscles forms a cuff(rotator cuff)which connects these bones and allows smooth movement..

Physiotherapy :
If physiotherapy and exercises injections of corticosteroid do not help,joint replacement may be advised .The prosthesis is composed of 2 parts:
1) The humeral component made of metal.
2) The glenoid component –this has metal tray which connects to the bone and a tough plastic cup which forms the socket.