They're just "storage" for data while the game or Bodyshop is running. Sort of like how your browser stores temporary data in your Temporary Internet Files and even in your internet cookies.
The Groups.cache is related to sims, genetics, and clothing I believe... I could be wrong about it, but that's my guess. :dontknow:
The Accessory.cache is related to just as its name suggests... the accessories for the game.
(Like I said, I could be wrong about these, but judging by the names, that's my take on them. :lol:)
