They tweak the proccess and make minor improvements to the CPU's as they are being made, that help improve clock speed, or add new features and what not. Thats what the differant steppings mean. A0 is like the very first revision, B0 would be the second revision. I believe a change in letter is a bigger change than a change in number. B1 would be a slightly tweaked B0, but C0 a bigger change over B0. Usualy there will be several revisions before it is ready for release, so the first CPU's out on the market could be a B0 or B1 revision, etc..