Mixing is the art of balancing and optimizing the various tracks/channels in a recording so they can combine coherently and enjoyably into a mono, 2-channel, or surround sound mix.
Mastering
Mastering is the final stage of the production process. A mastering engineer takes a high-level approach and looks at a song and/or project as a whole. This step provides the finishing touches.
Stem Mastering
Stem mastering is a process that lies in between mixing and mastering. Rather than manipulating all the individual tracks in a song, related tracks are grouped together and exported as stems.