i'm not completely sure about it myself but i know each photoshop cs has a version, so cs1 is version 8, cs2 is version 9, and it carries on like that with program improving as it goes up e.g. the cs up from the one below will have more features and things will have been improved where they've found glitches. so if you were to get cs5, it would be vastly different to the lower versions and i thiiink that's how it works