the above in the formation is cool for an MP3 file, anyone has a little idea about the encoding style of the .aac format, so I can get metadata information.
What programming language do you use? Do you just want a specification? http://www.danhinsley.com/metax/metax.html is this link the utility you are looking for? mp4 is a common container for aac content.
Java libraries for interacting with AAC (including tags) include:
http://www.fightingquaker.com/myid3/
http://jid3.blinkenlights.org/
http://www.jthink.net/jaudiotagger/