During a half hip replacement, doctors put a metal stem in the femur with a ball attached to the top of it and secure it to the femur, either by wedging it into the femur and allowing the bone to grow into it or securing it in the femur with cement. Once the ball goes back into the natural socket, it provides a very stable reconstruction of the hip.
Doctors like to use this procedure for a patient who's a very limited ambulator, who perhaps only walks around the house or a little bit outside. In this procedure the socket is not replaced. The advantages are that there's less blood loss with this type of procedure, the surgery goes quicker, and the dislocation risk of replacing half a hip is less than if the whole hip were replaced. The disadvantage is that some people will have pain in their groin from where the ball rubs on their natural socket.
Both techniques have advantages and disadvantages, but both allow immediate weight bearing.