The finale polish on a song. Brings a little more shine and loudness to a track. Once complete a song is ready for radio play, streaming, iTunes, and CD. All done with industry standard software.
Mastering is the finale step in the audio engineering process. Mastering brings the song to its most refined clarity and industry loudness. Once done, a mastered song is ready to be streamed/sold online, printed to CD, or played on the radio.