The hip is a ball and socket joint. The femoral head (ball) is at the top of the femur (thighbone) and the acetabulum (socket) is part of the pelvis.

The surfaces of the femoral head and acetabulum are coated with articular cartilage, which is very smooth. The hip joint is lined by synovium, which produces a lubricating fluid.
1 Femoral head (ball)

2 Femur (thighbone)

3 Acetabulum (socket)

4 Articular cartilage

5 Pubis bone
Around the rim of the acetabulum is a lip made of a mixture of fibrous tissue and cartilage (fibrocartilage) that is triangular in cross-section; this is called the labrum.The labrum serves to deepen the socket, improve the stability and seal the synovial fluid within the hip joint. The capsule of the hip joint joins the outer edge of the acetabulum with the femur and completes this seal.

The hip is a very stable joint that allows a great range of movement. Strong ligaments further stabilise the hip joint and more than twenty muscles around the hip provide movement.
All of these factors provide smooth, pain free movement of the hip joint.