Mixing and Mastering What's the difference?

I get this question in the studio lately. A lot of times people think that mixing is mastering and vice versa. Well it's not!

Think of a mix as something that's not final, it can be changed. Mastering is something that is final (it can also be changed) but it's not working from the same sources.

Mastering should sound like a finished product. A mix should sound like a leveled out song.

Wow, I almost confused myself!

I know it's hard to explain, but that's the best I can do at this time. I guess the best example is to listen to something someone did at a studio, that's a mix and listen to a cd from a store, that's a master!