Variable Bit Rate (VBR), as opposed to Constant Bit Rate (CBR),
is a more intelligent way to compress a video (or audio) file.
Some scenes which don't have much motion, or contain large areas
of solid colors, can be more highly compressed than scenes which
contain lots of motion and detail.
VBR adjusts the bit rate (compression ratio) as it encodes,
based on the scene to maintain a constant quality. This
results in a smaller file size (less disk space used, or
less bandwidth required for streaming).
Do a google search for VBR vs CBR if you want more technical
details than I provided.
--
-Bob
_______________________________
Microsoft MVP
Windows XP Media Center Edition
http://www.microsoft.com/ehome