In a nutshell, mastering incorporates all of the audio work done on the master output from a mixed song or album. While originally intended as the mechanical preparation step for putting music into its final media format for playback, mastering later began to include additional post-mixing audio enhancement work in order to optimize the sound specifically for the various intended playback formats (ex: vinyl, CD, digital, etc.).
“Using the technique adapted to a small instrument the man writes a whole world of the deepest thought and most powerful feeling. If I could picture myself writing, or even conceiving, such a piece, I am certain that the extreme excitement and emotional tension would have driven me mad.”