Where can I read about codec/encode?

To clarify, AAC and MP4 is the same codec. It's the file formats (with extensions) that are different. For example:
MPEG-4 AAC encoded by iTunes: .m4a
MPEG-4 AAC via iTunes Music Store (with a DRM that only allows iTunes/iPod playback): .m4p
MPEG-4 AAC encoded by Nero: .mp4

Here are some links with info:
http://www.iis.fraunhofer.de/amm/techinf/layer3/
http://www.iis.fraunhofer.de/amm/techinf/mpeg4/audio.html
http://www.mp3-tech.org/aac.html
http://www.apple.com/mpeg4/aac/
http://www.cd-rw.org/articles/archive/nero_mp4_aac_preview.cfm

Technically, one is superior to the other (and it has "had its time in the sun (http://linuxtoday.com/developer/2001010600306NWDTSW)"), but the best way for you to decide is to encode various tracks from your CDs and listen for yourself. I think the "tests" that people refer to are silly at best and skewed at worst.

(By the way, don't be fooled that one is more cross-platform than the other. Both can be encoded/decoded right now on Windows, Mac OS, and Linux.)