Mixing is where all the individual tracks are combined and blended to create a stereo mix. This includes equalization, compression, volume adjustments, adding effects, pitch correction and etc. When mixing is completed, you should be pleased with the way the song sounds and feel.
(Wikipedia) – Mastering
a form of audio post production, is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device (the master); the source from which all copies will be produced (via methods such as pressing, duplication or replication).
Here’s a video of what we believe mastering is. Cheers to Abbey Road for a great example.