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A shunt is simply a resistor of a very very low resistance, like a big wire. It carries most of the current of the ammeter, and the ammeter measures voltage across the shunt to find current.

For example, if you want to measure 100 amps, you won't find many ammeters that will measure that much current, so to make one, you would use a shunt. The shunt would be a resistor with a value of about .01 ohms. The resistance has to be low enough not to make much of a difference in the circuit (or the ammeter itself will cause the current to be low because of
it's high resistance).

If you put 100 amps across a .01 ohm resistor, by ohms law, Voltage=Current*Resistance, you get Voltage=100 amps*.01 ohms, so Voltage = 1 volt. So you use a voltmeter to measure the voltage across the shunt, and you have an ammeter. 1 Volt = 100 amps, .1 Volt = 10 amps, etc.

Most ammeters use shunts of one type or another, most times they are internal to the gauge and you can't see them. They use shunts because it's very hard to wire a meter with wire large enough to carry that kind of current, so the shunt carries the current and the meter measures the voltage across the shunt, which indicates how much current is present.

As to whether you like your ammeter or voltmeter better on a car, it's really a matter of personal preference.

http://lists.twistedpair.ca/pipermail/classic-mustangs/1998-August/012903.html

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