A voltmeter measures voltage or potential difference and an ammeter measures current, their names come from the units that they measure in, V and A.
So basically a voltmeter is very useful because it can be put in many places around the car but will register *all* the different power drains that are there, will tell you about your battery, alternator condition, how much of a drain the stereo or lights are etc.
You have to be much more careful with an ammeter cos it goes inline with a particular power line and so can only measure the flow of electrical current through that one particular section. So if you had it in the power line to the amplifier, it would tell you how much current it was drawing but wouldnt tell you whether the battery was about to die!