mrossen got it completely correct.
also, if you were to have searched the sound file formats forum, you would have realised this is the most discussed format in that forum, everything is covered there.
also, if you were to have read the links ***stickied at the top of this forum*** you would have noticed this in one of the linked pages:
atrac (.oma, .omg, .atp) - the newer style Sony proprietary format designed for minidisc use. It always has a .oma, .omg or .atp file extension. It is similar to mp3 and probably only useful if you are reading files from minidiscs or writing for minidiscs. ***Note most of these files are rights managed so you cannot open them in any software programs.***