What seperates "hi-fi" from "mid-fi" in cdp's?

I find that in my search for the right CDP I really don't know what, mechanically, seperates the good from the not-so-good. Although I understand which brands are more highly regarded than others, I don't understand why.
Along those lines, I would also really appreciate a very brief description of the primary working parts of the cdp and how they differ among "levels". For instance, how does the number of bits matter? Or is it just another useless audio stat?
Or, if I am asking a bigger question that I think I am, maybe you could direct me to a good reference for this issue? Thanks JB3