Can anyone explain mastering? What is it for and why should anyone care? Should I have all of my recordings mastered?
When mixing, you are making sure that the stuff you recorded sounds perfect on your studio monitors inside your quiet control room. When mastering, you are making sure that the mix that sounds perfect on your studio monitors, is going to sound decent in your home, your car at route 66, the crappy loudspeakers that came with your PC etc.
When mastering, (and taking it seriously), you listen to your mix on more than one set of loudspeakers, I normally listen to a pair of monitors, a pair of good hi-fi loudspeakers, a pair of cheap plastic crappy sounding speakers and a pair of headphones. I then take care of the dynamic range of the mix (depending on the type of music you could either add a compressor or a limiter that can just squeeze the dynamic range to a level you want). I also check whether I am happy with the low frequency balance of the mix, that depends on the loudspeakers that the mix was made with. Sometimes you need to do a bit of equalizing using a linear phase EQ (so that all the stereo-related stuff stays intact.)
Depending on the room where you mixed and mastered, it can even be the case that you add a very little reverb to the mix while mastering.
When producing an album with all seperately recorded tracks that are all individually mixed down, the situation can raise that there are substantial differences between the individual tracks. To some extent it makes sense to start seeing them as one group of related tracks, and use the techniques described above to make the sound of each track a bit more similar to the others.
There are discussions as to whether this step belongs to mastering or to the final stage of mixing, but adding dither signal, downsample and reduce resolution to prepare your track for publishing on for instance a CD. If I recorded on 96KHz/24 bits, I normally check every now and then if this stage is not messing up my recording while mastering my tracks.
A great book on mastering is "Mastering Audio - the art and the science" (second edition) by Bob Katz. I'm looking at my copy right now and remembering how much I enjoyed reading it, even if I don't have the equipment to do what he's talking about.