Juice is a frontend for mpg123 and other players. It provides a nifty dialog-based playlist editor.
In order to play audio files, two player interfaces is provided. The mpg123-specfic interface uses the remote protocol of mpg123 (This feature was introduced in version 0.59r in order to accommodate generic frontends.)
The other player frontend is generic and allows for any player to be used. This is useful if you do not want to limit the functionality of juice to MP3's. Please note that this generic frontend is still in early beta phase and can be buggy.
It basically allows you to browse through your files in a "Midnight Commander"-like fashion. The MP3's you select is treated as a playlist which can be shuffled, rearranged, saved, etc.