why we use different plates (230-220-110)V ....etc
also 50 hz & 60 hz
which one is better, 230 v or 110 v ?

why are we using 50 & 60 hz, why not for example 80 hz ?

Is the voltage that we get from power socket RMS voltage or Peak to peak voltage?

 

You can take a look here http://en.wikipedia.org/wiki/Utility_frequency It may give some answers. Unless noted you should assume that AC voltages are given in RMS. And that is also the value that common voltmeters will show. As an aside. Most voltmeters are only good for measuring pure sine wave AC up to around 500 Hz.

 

why we use different plates (230-220-110)V ....etc
also 50 hz & 60 hz
which one is better, 230 v or 110 v ?

why are we using 50 & 60 hz, why not for example 80 hz ?

Is the voltage that we get from power socket RMS voltage or Peak to peak voltage?

The question of which one is better has no answer unless you indicate what constitutes "better".

Which is better, a black car or a white car? I could describe one set of concerns and you would say, "black"; but I could describe a difference set of concerns and you would say, "white".