An ''A'' list celebrity are the celebrities with a very high social status, very known and make the big bucks. (Drew Barrymore, Miley Cyrus (unfortunately), Jim Carrey, Justin TImberlake, Megan Fox,etc,etc.) A ''B'' list celebrity are the ones that aren't as popular, not as known.. basically the ones you won't exactly see on the cover of magazines. They might have been popular before, but not anymore lol.. Andy Pick (but with a D) referred to himself as a 'B' list :)
A list are the higher paid more popular celebrities for example - Angelina jolie, Brad pitt, etc. B list are the celebrities that are known, but not as popular. Margaret Cho, Kathy Griffin, etc.
This is all correct, but as far as I know the list extends to "C list" and "D list" celebs too. The idea being the same - the lower the letter, the lower your celebrity status