AAC

As there is MP3, there must be a MP2 and a MP1.
Indeed they do exist. Of course an MP3 player plays all the previous versions.
One day, they decided to drop this backwards compatibility.
Basically, if all knowledge obtained while developing MP3 could be used without the historical ballast what would be the result?
That is AAC, Advanced Audio Coding.
It achieves transparency at a lower bit rate than MP3.

 

If you are in need of a lossy compression (portables), AAC is a better choice than MP3.

 


Other differences:

References

  1. Advanced Audio Coding - Wikipedia
  2. HTML5 AAC Audio Playback Tests -  Fraunhofer