what is ogg vorbis

Vorbis is a free and open source, lossy audio codec project headed by the Xiph.Org Foundation and intended to serve as a replacement for MP3.

Usually Vorbis coded audio files are stored in Ogg type files (which is basically a container file type which can contain many type of info including video and audio), that's why we usually talk of "Ogg Vorbis" files.

Unfortunately Ogg Vorbis format is not widely supported on portable MP3 players, and it's a shame because it offers the same quality and performances of the MP3 file, being totally free and open source.