The cartridge inside a faucet is a type of valve or a part of the valve. It is, in fact, an improvement over the old compression type valves that are harder to operate using one hand. Cartridges have a few advantages over the old type. The most notable one is that it is easier to operate a cartridge faucet. You can open or close them by turning the lever ninety degrees. They are also much easier to replace when they break. All you have to do is get a new cartridge of the right specification and replace the old one with it. But if you see a faucet leaking, first check the washer around the cartridge before replacing the cartridge.