Also, most meters measure the AVERAGE value and display the result as RMS. Peak measuring meters were pretty much limited to vacuum tube voltmeters and most of these also had peak-to-peak scales on them as well as RMS scales so that readings could be correlated to scope measurements in the TV repair industry.
In all cases, non-true-RMS meters assume a sine-wave input for their accuracy.
Regarding the frequency response issue: I've checked some Fluke DMMs to find that their upper end rarely made it above 1KHz, others going to 10KHz or more. I've checked the venerable Simpson 260 VOM to 100KHz and found it pretty decent! AC voltmeters (they measure AC only, usually down into the millivolt level) usually typically have frequency response somewhere between 1MHz and 10MHz, depending upon whether they come from Asia or Hewlett-Packard/Agilent. RF millivoltmeters have responses extending beyond 1GHz, although limited in maximum voltage input.
Dean