iTunes 11: AAC
Advanced Audio Coding (also called MPEG-4); a standard way of compressing and encoding digital audio. AAC files can be played using iPod, iPhone, iPad, and applications that support QuickTime.
AAC-encoded files rival the quality of audio CDs, and sound as good as or better than MP3 files encoded at the same or even a higher bit rate. For example, a 128 Kbps AAC file should sound as good as or better than a 160 Kbps MP3 file. Because the bit rate is lower, the AAC file will also be smaller than the MP3 file.
AAC files enable you to store the most music on your hard disk or device. At the High Quality setting, AAC files use approximately 1 MB of disk space per minute.