Ogg Vorbis is a lossy codec, meaning audio quality is lost whilst compressing (how much is lost depends upon bit rate used, higher bit rates will produce audio sounding exactly as the original). Ogg Vorbis is comprised of two parts, Ogg is the container whilst Vorbis is the compression format. The default quality of 0.7 (estimated 224 kbps) gives very reasonable encodings and should be a starting point.