The image at the bottom of the page shows all of the parts most commonly found on two and three valve bath and shower faucets. This type of faucet is available in both 8" and 11" centers. Almost all newer bathrooms use the 8" model. To check your size, measure between the center of the hot, to the center of the cold handle. 

Leaks

A leaky faucet is usually easy for the do-it-yourselfer to repair. In most cases, no special tools are required. In some situations, a special socket wrench and/or a seat tool may be needed. Below is a list of tools you may need. 

The first step is to shut off the water leading to the faucet. Next, remove all the trim. The trim is usually chrome or brass plated and includes the handles (3) and escutcheons (4), which are either tubes and rings, or cones that hide the holes in the wall. To remove most handles, take out the screw (2) in the center and pull off. Sometimes the handles will be stuck on from soap. If so, it's best to use a handle puller. You can gently tap, or pry them off, but you risk bending, or breaking the stem (10 & 16). Some handles, usually fits-all types, may be held on by set screws, accessible from the side.

The next step is to remove the escutcheons (4). The cone type almost always screws off. If your escutcheon looks like a tube with a separate ring around it at the back, look for a hole on the edge and loosen the screw beside the hole and pull off. You can now unscrew the tube with your adjustable wrench. To keep the tubes from being scratched,  wrap in electrical tape first. Almost all faucet valves have a packing nut (6) in them. You will usually see what looks like a similar nut inside of a large nut. It is best to leave this nut in while doing the next step. If you have a cone escutcheon, the packing nut may come off with it. Try and remove it and replace in the stem bonnet (8 & 14).

Using a pipe or adjustable wrench, screw out the bonnet. DO NOT use a vice grip wrench. It will probably crush the bonnet, which is made from soft brass. In most cases, the stem will come out with the bonnet. If not, just screw it out. If the bigger nut is behind the wall, you should use a Shower Bonnet Socket to remove it. You should replace all washers and gaskets while you have things apart. Replace the bibb washers (11) and bibb screws (12) on the back of the stem. If the washer is torn, loose, or missing, you should replace the bibb seat.(13) which is in the very back of the hole that you took the stem from. There are a very few that do not use seats. Most hardware stores that carry faucet stems will also have a selection of  faucet seats. Be sure to use the correct seat. Remember, you wouldn't put a Ford part in your Chevy!

Replace the cap  thread gasket (9) on the back of the bonnet. If you had a lot of trouble removing the bonnet, the cap gasket was probably missing. Replace the packing (7) in the front of the bonnet. Be sure to remove the old one first. It's probably inside the bonnet and can be removed by plucking it out with a small screwdriver. It's best to use the correct, pre-formed packing for your stem. Self forming string packing  should only be used if you cannot get the correct packing washer. In all cases, the packing should be installed over the stem, after it has been inserted back into the bonnet. Screw the packing nut in HAND tight. If your stem has no packing, you will probably find one or more "O" rings that should be replaced. It's also a good idea to coat the threaded area of the stem with plumbers non-toxic waterproof grease. This will make the faucet easier to operate and reduce wear. Also note, on 3 valve models, your center control will be different than the hot and cold and will usually need some other special washers called Bath Diverter Washers.

Now, let's put it back together. With the stem in the bonnet, in the full open position (screwed as far up to the back of the bonnet as possible), screw the assembly back into the valve body. Using your adjustable wrench, tighten it up ,but don't over do it. DO NOT REPLACE ANY TRIM YET. Push a handle on and turn the valve so the water will be off on both hot and cold. Turn on the water supply. Slowly turn on the valve that you repaired. You will probably see water leaking around the packing nut. With your wrench, turn it just enough so it stops leaking. Check for leaks around the bonnet, and snug up as needed. If all this is well, replace the trim, and enjoy the rest of the day. Some companies now offer complete kits that include everything you need, (stems, handles, escutcheons, seats and the bonnet wrench), these kits are available to fit most major brands.