The hip joint (“a ball and socket”) joint is made up of 2 bones:

The pelvis which has the socket(acetabulam) The thigh bone or (femur) whose head forms the ball The bony surface are lined by cartilage.

Wear and tear of this cartilage causes the exposed bone ends to rub against each other producing pain,limitation of movement and deformity.

This wear and tear may be due to:
Arthritis
Avascular necrosis (loss of blood supply) , Infection , Fractures etc.

Hip Replacement may be partial(only femoral head is replaced) Or Total (the entire joint surface is replaced)