ok I will pretend to be from rio linda, AAC?
Advanced Audio Coding. You may know the old MPEG1 and MPEG2 media standards (the latter used on DVDs), and MP3 is just the audio portion of these standards. Quite old-tech.
AAC is the audio portion of the newer MPEG4 standard, the video portion of which is used in the new high-definition video formats Blu-ray and HD-DVD. AAC delivers higher quality for the same file size (bit rate), can do twice the sample frequency (how many samples per second of the music), handles high-frequencies better (MP3 loses it at 16 KHz) and can hold more channels.
Right now the program most famous for using AAC is Apple’s iTunes because it’s set to AAC by default and the iTunes store sells only AAC files.