The well-fitting surfaces of the femoral head and acetabulum, face each other.

http://en.wikipedia.org/wiki/Acetabulum

The well-fitting surfaces of the femoral head and acetabulum, which face each other, are lined with a layer of slippery tissue called articular cartilage, which is lubricated by a thin film of synovial fluid. Friction inside a normal hip is less than one-tenth that of ice gliding on ice.