Digital Voltmeters (DVMs) are a special case of A/Ds. DVMs are voltmeters - i.e. they measure voltage - and are general purpose instruments commonly used to measure voltages in labs and in the field. DVMs display the measured voltage using LCDs or LEDs to display the result in a floating point format. They are an instrument of choice for voltage measurements in all kinds of situations.
Obviously, if voltage measurements are taken and the results are displayed digitally with LED or LCD displays, the instrument has to contain an A/D converter. Digital voltmeters have some characteristics that you might need to understand.
Digital voltmeters usually have scales that are 0-0.3v, 0-3v, 0-30v, 0-300v, etc.
It is not clear why those ranges were chosen but they are commonplace. Now, consider some of the implications of these facts.