The cartridge is the valve mechanism inside your faucet that controls flow and temperature. When the faucet drips, the handle is stiff or loose, hot and cold do not mix right, or water leaks under the handle -- the cartridge is almost always the problem. Unlike old compression faucets with rubber washers, cartridge faucets use a self-contained unit that slides in and out as one piece. Remove the handle, pull the old cartridge, push the new one in. The catch: every brand uses a different design, so you need the exact replacement for your model. Moen, Delta, Kohler, Pfister, American Standard -- all proprietary.