mixing is to get the RELATIVE levels of each instrument and
their effects right.
DIGITAL MASTERING (for a SINGLE TRACK) is used to take an
ALREADY MIXED TRACK, EDIT start and
stop times and initial and final silence, do a touch up eq over
the whole mix, exciters
for different levels, compression whether single or multiband
compression to even out the levels thoughout the mix, control
the stereo image for different frequency bands, LIMIT the out put
so that it doesn't go over unity or other volume setting near unity,
without clipping or distortion but still introduce gain in ROOT
MEAN SQUARE loudness, and finally introduce dithering if necessary
if bouncing your track from a higher sample rate bit rate audio
to standard cd audio 44.1 kHz 16 bit audio file.
There is also tape mastering and vinyl mastering which often require
more brutal eq curves to compensate for the medium they will
be mastered to. Mastering technically also includes the manufacturing
of a physical template that will be used to make copies of the media,
so there is such a thing as CD mastering BEYOND the digital process,
i.e. making the cd stampers (I used to work at a cd/dvd)
mastering/duplication place part time)

Peace.