Uses technology built into MPEG-2, the system used to make DVD movies. Should provide better sound and better compression than MP3. It's being tested by EMI and BMG for distribution of music. It's a nightmare of licensing. Apparently it doesn't quite work right.
Nobody expects it to catch on anyplace else.
Used for low bit-rate voice. Not expected to be used in music. This is the next video standard and incorporates a high-quality audio compression system that would naturally supplant MP3. Fraunhofer and Microsoft both have patents that would need licensing. This is an open-source system that is supposed to be superior to MP3 and have no associated licensing fees.