An ammeter wires into your electrical system in series, meaning you must run large wires to the gauge to support most (if not all) of the truck's electrical usage. An ammeter is like a flow gauge for your electrical system, and measures amps. It will tell you how much electricity you are drawing or putting into the battery.
A voltmeter wires into your electrical system in paralell, and is similar to a "fuel guage." A dead battery is close to 12.0 volts, a fully charged battery is 12.6volts, and anything from 13.0-14.5 volts is charging range. A voltmeter is more usefull for monitoring the state of your electrical system, IMO. The explorers come with one std.