There are many different models of implants. Most are made of metal and plastic, with some newer ones using metal-on-metal or ceramic.
In a hip implant, the cup component that replaces the damaged hip socket is most often metal with a plastic lining. The ball that replaces the damaged head of the thighbone is most often metal. The ball has a stem, which fits into the middle of the thighbone, and this stem is also metal.
In a knee implant, the piece that replaces the damaged end of the thighbone is usually metal, and the piece that goes on the kneecap is usually plastic. The piece that replaces the damaged end of the shinbone usually has a plastic piece sitting on top of a metal tray. The tray has a metal stem that fits into the middle of the bone.
Some implants are put into place with bone cement. Others are made of a porous metal that lets your own bone grow into it.