It is now considered to be the actual "state of the art" in general audio coding and the natural successor of MPEG-1/2 Layer III / MP3 in the new multimedia standard MPEG-4 that uses MP4 as the container format for all kinds of content.
AAC is able to include 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 15 low frequency enhancement (LFE, limited to 120 Hz) channels and up to 15 data streams. Besides it has further multi-language capacities.
MPEG formal listening tests demonstrated that AAC provides an audio quality at 96 kbps which is slightly better than MP3 at 128 kbps and MP2 at 192 kbps.
AAC means "Advanced Audio Coding", and in the beginning it was also called MPEG-2 NBC for "Non-Backwards Compatible" as opposed to the MPEG-1 and MPEG-2 BC (with 5.1 channels) standards.