There is usually a ring or faceplate that surrounds the cartridge. This will need to be removed to access the shower faucet cartridge. You can remove this by turning the ring with a standard wrench. You may need to use a knife to cut a line into any caulking that is around the ring.
Not all shower faucet cartridges are the same. Take the old cartridge to your local hardware store for a direct match. You will see the wear on the old cartridge compared to the new one. Wear and oxidation on an old cartridge causes water to leak around the area.
The shower faucet cartridge is the piece of pipe that connects the handle that changes the water supply from tub to shower with the inner valve. If the handles in your shower do not turn the water all the way on or off, the cartridge needs to be replaced.
A shower faucet cartridge is one of the faucet's most important parts. This part is the very reason you are able to control both hot and cold water with a single handle. Most modern faucets consist of a cartridge and handle design, instead of the valve and stem design common on older faucets.