A knee replacement is really a bone and cartilage replacement with an artificial surface. The knee itself is not replaced, but rather an implant is inserted on the bone ends. This is done using a metal alloy on the femur and a plastic spacer on the tibia and patella (kneecap). This creates a new, smooth cushion and a functioning joint that can reduce or eliminate pain.
A hip replacement is an operation that removes the arthritic ball of the upper femur (thighbone) and damaged bone and cartilage from the hip socket. The ball is replaced with a metal ball that is fixed solidly inside the femur. The socket is replaced with a plastic or metal liner that is usually fixed inside a metal shell to create a smoothly functioning joint.