Mixing, as the name implies, is mixing the relative balance of your individual tracks in volume and tone (through EQ, reverb etc).
Mastering - it helps to have a little history. It is maximizing the benefits of going from one medium to another. As an example, going from a 24 bit mixdown "master"to a 16 bit/44.1 sample rate copy for CD replication. In the old days of vinyl, that medium couldn't reproduce the full range of tape. Lots of bass or volume would bounce the needle out of the track, for example. So they ran the tape output through things like the fairchild limiter to compress the volume and rolled off the bass eq with pultec eqs and such. These are still used 40+ years later and you'll pay high dollar for them.
Like tracking and mixing, mastering turned from a science (the guys in white coats at Abbey Road) more into an art. Bands recorded in different studios, with different people, and part of "mastering" became less of matching volumes between songs on an LP and making the entire production cohere.
Now days mastering is less of a final polish to a song or CD and more a matter of making it louder - AS LOUD AS POSSIBLE. A little science sez that people prefer a song or sound slightly louder, and therefore it seems a good bet to make your song the loudest that will be heard. Which is self defeating, since radios and TV have their own limters/compressor etc, so you are flatlining a flat line. At home, it means reaching for your volume knob
As a practical matter for the home recordist, it means adding a final polish on your work, raising the overall level of a song and matching song volumes on a CD so your one ballad on your rock opus doesn't sound like you hit the dim button on a console and leave the listener scrambling for the volume remote. Mix to -12 dB, or -6, or even -3 if you are rocking. Buy SF studio ($50) and some mastering tools (Voxengo less than $100) or just use SONAR Producer's stuff to squeeze a few more dBs out of your songs and gentely put an EQ curve on all the songs. Voila, mastered.
As you work you will get better (if you have any intelligence and/or talent). But there is a reason people pay other people to master - it is a science, art and craft and some mastering engineers are better than others. Phil York, who just died last week (won grammys- engineered Red Headed Stranger by Willie, for one) was a whiz with the TC Konnekt Finalizer. It is considered passé today, but he would punch the buttons quick as a feather-weight fighter and as good of a master as one could expect would come out the other end. You could give me and a million monkeys a room full of the finest analog equipment and it never would have sounded as good. There is something to be said about spending your life learning to do something.
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