minidiscs (MD) is mainly a format to hold audio. you can put mp3's and wav's on them, but what you are doing is just recording onto the MD's in real time. sony has come out with netMD, which i believe allows you more flexibility to download mp3's on MD. it just came out, so i'm not really up on the specs and features.
you can't really store anything else on an MD. there is a Data MD format, which are MD's used to store data and such, but you need a special Data MD drive (which i don't think sony makes anymore). also, sony has an MD camcorder that uses the discs.
for more info on MD, take a look at this site:
http://www.minidisc.org/
that site should be able to answer any questions you have.
tr