VBR is pretty nice as it expends more bits on complex parts of the track while it "saves" bits on the simple (to encode) parts instead of using the same bitrate, however complex or simple the various parts of the track are.

With VBR you can also, if you want, set a minimum bitrate that the encoder will never go below, even if it would still provide acceptable results for that part of the track with an even lower bitrate.

You can also use ABR - Average bit rate - where the encoder juggles things so that the file size, while the encoder still uses variable bitrate, still comes out to the same size as if you had encoded it with a fixed bitrate (it just expends more bits on the complex stuff and less on the simple). Useful if you want more control over the resulting file sizes.

/Michael