Mastering is not quite the simple process of deciding on the final stereo version, which I think of as mixing. Mixing can be done in the studio by the engineer and band, where they make decisions about the sonic qualities of the different tracks (e.g treble, bass, reverb; how much volume they have vis-a-s ) as well as placing them in a stereo space (i.e which instruments go where, and what's the stereo separation - i.e. to what extent are they mixed in the center, or are they totally separated into right and left sides). Once this final mix has been done, then it is mastered - which is a process of doing SOMETHING to the sound so it can be reproduced faithfully. I believe it has something to do with creating qualities of muddiness or clarity. I've heard of good mixes getting poorly mastered, and thus sounding terrible. (this is where that technical link comes in.) When my band produced a CD, we recorded and mixed the tracks with one fellow, then took it to another engineer to master. I'd ask to hear some work products by the guy that's going to master it, and listen to them on the CD you're accustomed to. Do they sound crisp and professional? That's the ticket!