"A" is usually the first revision of a cpu speed, it could mean the cpu has a 400mhz fsb, while a "B" version might have a 533fsb, or a "C" version might have a 800fsb, and an "E" version is a version has the newer "prescott" core, and more cache(1mb, vs 512kb). Sometimes an "a" can be 533fsb though. It's complicated, say intel has a 3ghz chip on a 533fsb, if it's the first 3ghz chip it's likely an "a", if they then come out with a 3ghz cpu with a 800fsb it could be a "B", and if they have a 3ghz prescott core, it's an "E".
C is almost always a 800fsb, and an E is always a prescott. A and B can vary.
EE is the version for "extreme edition".
Example, a 2.4A 400fsb, 2.4B 533fsb, 2.4C 800fsb, 2.4E prescott. All 2.4ghz, different bus speeds.
Intel is just confusing.