The diagram shows how to wire a Delco (GM) alternator. Note that this applies to the older 3 wire Delco alternators, not the newer (~1985+) Delcotron electronic alternators.

1. This is the main power wire that goes directly to the battery. Make sure it is a heavy gauge since it has to carry the full output of the alternator (up to 100 amps).
    

2. This is the field wire that energizes the alternator. It must be switched, or else it will drain the battery overnight. Do not wire this into the coil power wire that you use to switch the engine on and off (a seemingly elegant solution), since once the engine is running, the alternator will power the coil, and your ignition switch will no longer kill the engine.
    

3. This is the lead for the sensing wire that goes to the idiot light in your instrument panel. A neat tip: if you aren't using a voltmeter or oil pressure gauge, run this wire to a light mounted somewhere on your dash. Often you can't hear your own engine running in the derby, so when this light lights up you know that your engine has just quit (or the alternator has just stopped charging for some reason).
    

4. This is the regulator bypass hole. Sticking a long thin metal object about 2 inches deep into this D-shaped hole causes the alternator to go to full charge. If you do this while it's dark out and the main power wire is disconnected, the whole alternator will emit a cool X-Files type glow. I don't recommend that you try this since you'll likely end up frying your alternator.