This is merely a visual representation of common PC screen sizing (pixel width) as compared to Video Encoding sizing (e.g. "Widescreen" and DVD sizing).
Click the image to enlarge to it's actual size. This preview image is 640 x 480 pixels (width by height).
A clear explanation of Movie sizing can be found at Widescreen-O-Rama! (All About Aspect Ratios)
See also Doom9's explanation as it relates to re-encoding.
this page, initial post: 2005 Feb 18;