Mastering should be (and always used to be) providing small changes to a stereo mix to ensure that the album is consistent with other albums in the same genre. Hence the role of the Mastering Engineer as a specialised audio professional: he / she is the person who a) has golden ears, b) has an excellent listening environment (room and speakers), and c) has vast experience of what a mix should sound like due to listening to many similar mixes from multiple studios.
Fast forward to today and every wanna-be pup engineer with ProTools LE wants to master his own recording. For an average band recording this will entail multiband compression, limiting, equalisation, and adding dither as the mix is bounced from 24 bit to 16 bit for CD.
The usual result is to produce a final mix that is over-compressed (and harsh sounding), and as the level rarely leaves 0dBFS dithering becomes pointless.
Dave Moulton has a good article on mastering here.