The recording below starts off with the mixed version, then changes over to the mastered version.
The most obvious difference is that the mastered version is much louder. Tracks need to be louder to compete in today's marketplace, but the skill lies in getting a track loud, but without falling into the loudness wars and squishing the life out of it.
The guitars also sound much wider, the bass and kick drum thump harder, everything sounds bigger and fatter. The track sparkles and is much more exciting and hard-hitting.