Purpose of Automotive Voltmeter Gauge

I've got a vehicle with with a standard voltmeter gauge, and while I've been driving for over 40 years now, I'm not sure I know entirely what the increments on the gauge face really tell me.

In less-nice vehicles, I've had "Alternator" lights that illuminated, when there was problem, and that was typically when a belt that drove the alternator broke. I'm not having any problems, so this is more just an intellectual curiosity issue - although I suspect any explanation provided will say something about under-charging or over-charging (although I'm much less clear about the circumstances that would cause the latter). Thanks!