Mastering doesnt do anything to any physical media, its just the process of making demos (or any unmastered song) sound up to par with other commercial records. The only physical part is that you make one CD ready for duplication (hence the "master copy"), but I dont do any mass copying.
For example, if someone were to send me a CD, I would just rip the audio files to wav and edit them then. Or you could just post them on the web (even mp3s...I can convert back forth without losing any quality) or email them to me and I could send them back to you the same way.
If you want really really high quality you have to go to a mastering house, which uses $100,000 worth of equipment.
