Mixing and mastering often to hand in hand, but the two processes aren't the same thing. Mixing is done first, and mastering is done afterwards.
Mixing involves balancing out different elements of a song. This can include EQ, Compression, effects like reverb / delay, and any other processing. Mixing often involves making changes to the individual elements inside of a song, like the drums, vocals etc.
Once a song is mixed that track then needs to be mastered, and this is what eMastered's mastering engine does. It takes your mixed song and polishes it, making it ready for commercial release. The mastering process handles the entire song as a whole.
In summary, mixing occurs first and focuses on individual elements of the track and making them sound balanced together. Once mixing is done the song can be mastered, and in the mastering process the file itself is processed as a whole rather than in individual parts.
We have written a blog post as well that highlights what mastering is and how it differs from mixing, you can read it here: