Mixing is when you take all the various recorded tracks and you realize they're all recorded at different levels and you maybe want to put different effects at different points on different instruments or voices and you start tearing out your hair and getting depressed because it sounds like a mush and you burn lots of CDs and run out to the car to listen to what you've done and it sounds awful even though it was awesome when you tested it on the computer etc, etc, etc, and the night ends with you selling the whole lot on ebay and kicking the cat or your kids or the postman and cursing the Queen, the President or Fleetwood Mac and then you break down in tears wondering why God isn't answering you right then........
then you wake up and discover it was all a dream. And you take all your recorded tracks and blend them in a way that makes the song in question easy to hear, pleasant to the ear and hopefully, interesting. Your song should be mixed before you get it mastered, if you're going to get it mastered.
I think it should sound good before mastering, which should make it sound better. Mastering sure ain't mixing ! I guess it's a process that enables the mix to sound at it's best at all volumes on whatever machine that mix is played on.