Or mastering is a process which a good track is adapted to sound the same or more enhanced when put to vinyl. Bear in mind the sound characteristics changes from a vinyl, at the outer rim treble (or bass, I can never remember) is slightly distorted and near the spindle the bass (or treble) is distorted. On a good system I've heard it say you can really hear the difference.

So a good mastering engineer will take this into account and modify the sound output to fit a vinyl better and produce a superior sound rather than just a straight to vinyl press.

So if you are just going to release stuff on mp3's or whatever it's a bit pointless.