The thighbone, femur and the pelvis, acetabulum join to form the hip joint. The hip joint is a "ball and socket" joint. The "ball" is the head of the femur, or thighbone, and the "socket" is the cup shaped acetabulum.
The joint surface is covered by a smooth articular surface that allows pain free movement in the joint.
The cartilage cushions the joint and allows the bones to move on each other with smooth movements. This cartilage does not show up on X-ray, therefore you can see a "joint space" between the femoral head and acetabular socket.