mixing is putting all the elements of the song(vocals, snare, kick, bass, lead, guitar) into seperate tracks.....and using panning and volume to each track correctly so that no element drowns out the other.....
mastering is another story......ever notice how comercially released cds sound more polished than the cd that youre homie did in his basement?....mastering gives recordings a more polished, ready for radio sound.....
hope that helped.....i was trying to keep it shot and to the point