MP3s get a lot of headlines, but chances are you have more AAC files than you may realize. The prevalence of AAC comes down to one major factor: Apple's iTunes software. All iTunes song purchases come in the AAC format, and unless you've tweaked some settings in the software, all the music you've ripped to iTunes from CD is probably in AAC, as well. To determine if a song is AAC, see if the file ends in the .M4A extension.
Advanced Audio Coding (AAC) files have a few technical advantages over MP3, most notably their superior efficiency at encoding music compared with MP3 files at similar bit rates. Still, AAC files aren't as universal as MP3, and would likely be a bit player if not for the overwhelming popularity of Apple's AAC-friendly iPod hardware and iTunes software.