There are a number of different types of AC-voltage measurement (peak-to-peak, RMS, etc.), and they'll generally yield different values for any given signal. In many cases, if one has a measurement of a known type and one also knows the shape of the waveform and DC offset (if any), it will be possible to compute what the other measurements would have been (e.g. for a sinusoidal signal with zero offset, the peak voltage will be about 1.414 times the RMS voltage), but a number by itself, without information about what kind of measurement it represents, is apt to be meaningless.
For many purposes, sinusoidal waveforms are reported as RMS voltage (a 120V or 240V power main, for example, will nominally have 120V RMS or 240V RMS), but cheap meters will often measure AC voltage via some other means and then scale the result in whatever fashion would be appropriate for a sinusoidal signal with zero offset.
If one is measuring a sinusoidal signal with zero offset, such a meter will work just fine. In other cases such a meter may still be usable (and in fact may sometimes be better than a true-RMS meter) if one knows how its measurements are computed and can figure out from that what one wants to know about the signal (e.g. if one has a meter that is known to measure the peak voltage and scales it by 70.7%, and one wants to know the peak voltage of an irregular signal, one could use such a meter by multiplying its displayed result by 1.414, while an RMS meter may be nearly useless).
The primary advantage of a true RMS meter is that it will measure irregular waveforms in a known fashion, subject to documented frequency restrictions. Other kinds of meters may perform measurements in ways that would be sometimes more useful and sometimes less useful, but unless the meter documents the actual measurement techniques used, they're apt not to be useful at all.