When you import songs into iTunes, it gives you preference options I think they are: [copy the file(s), move the file(s), let me organize my music folder]
When you let iTunes manage the music, it categorizes them into folders by artist and album.
Sometimes the digital signature of a CD gets mixed up with another (i.e., my brother's band has a CD that when you let iTunes import it and name the songs, it thinks it is an Iron Maiden CD from 1987).
There are a few different apps for OSX that you can use to fix your tags:
Fix Tunes : www.fixtunes.com
and one called
I Eat Brainz: http://musicbrainz.org/doc/IEatBrainz
I have used both, and both work well, however ... there are still some tags it cannot retrieve; mainly for Mp3s I ripped LONG LONG ago.