speedy

Hi all, I am new to aircon and like to know what is an invertor and non-invertor. And what is a system 3 versus a system 4? I like to put aircon in my 3 bedroom and all 3 of them using for the whole nite. Is it better to use invertor or non-invertor? Actually I like to put one in the living room. But is it going to be very expensive, when comes to electriciy bills? Thanks.