Tub and shower faucets work much like sink faucets, but they are oriented horizontally rather than vertically and their parts are usually larger. A two-handle stem shower faucet has a stem with a washer that presses against a seat to seal off water. A three-handle unit adds a stemlike diverter to direct water up to the shower or down to the tub spout.
Getting the parts
Often leaks can be fixed simply by replacing the washers. Even if you need to replace the seats or stems, it's easier to repair than to replace an old shower faucet, because replacement requires opening the wall. If parts are hard to find, order them at a plumbing-supplies store.
An hour or two for most repairs
Screwdrivers, handle puller if needed, adjustable wrench or groove-joint pliers, stem wrench or seat wrench if needed
Shutting off water, dismantling a faucet, installing small parts
Shut off the water, close the tub stopper, and place a rag in the tub to catch any loose parts
Washers, O-rings, seats, new stems if needed