Supposedly the successor to MP3, the widely-used music compression format. MP3pro was released on the 14th of June, 2001. It is a joint project by Thomson Multimedia and the Fraunhofer Institut für Integrierte Schaltungen, the inventor of the original MP3 format, and its patent holder.
The problem with the MP3 format is the fact that encoded files containing many high-pitched sounds (crash cymbals, hi-hats, some orchestral instruments) are audibly distorted. MP3pro promises to eliminate such distortions by adding an extra audio channel to the file, one specifically designed to deal with high-frequency signals.
The new codec is also backwards-compatible with the old MP3 format, allowing it to play them also. Conventional MP3 players (hardware/software) will have trouble with the 3rd channel, but will supposedly be able to play MP3pro files.
Another improvement over MP3 will be the stronger compression. Now, a MP3pro encoded file at 64kbps will be the equivalent of a standard MP3 file at 128kbps, effectively halving the size. Luckily, the MP3pro format doesn't support any copy protection, just like the original, at least for now.
Anyway, it remains to be seen if CD-ripping programs and file-sharing utilities will jump on the bandwagon and support the new format.