John, mixing and mastering are two different tasks usually done by two different people in the "real world".
Basically (pre-)mastering means taking a mix and making it sound consistent and "loud" using sum compression and equalization (and some more detailed techniques, if necessary). The plain mix should usually not have any sum procession at all and thus have a large range of dynamics (i.e. the difference between the lowest and the highest volume peaks throughout the song should not be too small on average). So the mix will sound rather low in terms of volume and loudness.
Compressing the material will decrease that dynamic range, making the mix sound tighter and denser. Since compression also reduces the volume, the compressed material is then brought back to the original peak volume, but the result will sound louder than the original mix since the density is more consistent.
There is much more to mastering, but that's the basic goal of pre-mastering a song.
The whole mastering task though is not just dealing with one separated song but with a whole album, so mastering also includes bringing the songs in a decent order that "works", and adjusting all the songs to a consistent level.
Mastering an album without an EQ would be impossible because if the songs on an album have completely different frequency response characteristics the album would not sound coherent.
Anyway, for us hobby guys mastering usually just means sum compression to make the song competitive in terms of loudness and punchiness.
Regards,
Steffen
