Ogg Vorbis (sometimes just called Vorbis) is an open source patent-free audio compression format, developed as a replacement for proprietary digital audio encoding formats, such as MP3, VQF, and AAC. Vorbis files (which have an .ogg extension) compress to a smaller size than MP3 files, which reduces bandwidth and storage requirements. According to many reports, Vorbis provides better sound quality than MP3.
To encode OGG Vorbis audio with River Past software, please have either Torbias' OGG DirectShow Filters or Radlight Ogg Media Filters installed.