Explanation
-i
is to specify the input file, which can be any kind of video file, such as mpeg, mkv, flv, and others.
-vn
means "no video", effectively extracting audio only.
-acodec libvorbis
specifies to encode the audio as ogg. To encode as mp3, use -acodec libmp3lame
instead.
See more related tips at this gist.