No offense to autozone, but their test are only 50% accurate and if you think about it, so is guessing:-)  The sure fire way to determine wether or not your battery or your alternator is dead is simple and fast.  Start your engine (get a jump from a good running vehicle if necessary).  With the engine running, disconnect the battery.  If the engine dies-your alternator is dead, if it continues to run, your battery is dead.  The reason for this is that the battery is used only to start a vehicle and occassionally to assist in accessories (headlights, stereo, AC, etc...) during idle or traffic stop.  The alternator keeps the fuel injectors and ignition system (onboard computer, etc...) going once the vehicle has started and begins to recharge the battery from the energy used to start the vehicle.  So if your engine still runs after disconnecting the battery, the alternator is doing its job correctly.  If it dies, then the battery was the only thing supplying the ignition system and everything else.  Please know that 90% of alternators die due to a failing battery.  So if it proves to be the alternator, you may want to replace the battery as well, unless you're sure it wasn't the battery due to age, etc...

  Please note:  Some Ford Vehicles have an external regulator that may be dead, instead of the alternator.  The best way to diagnose this is to start the vehicle and use a voltmeter + side on the single large wire/cable on the alternator, with the engine running it should be putting out 13-14V.  Then test the leads coming off of the regulator.  If you have lots of voltage at the alternator, but not at the regulator, then it is the regulator that died.  Autozone has more accuarcy diagnosing the regulator on these vehicles.