Beginning in season four, the Region 2 and 4 DVDs include a widescreen (16:9) version of the television show. The Region 1 DVDs include the standard format (4:3) version, which is identical to the broadcast version.
The reason for this difference is that the Region 2 and 4 DVDs were released before the Region 1 DVDs, with less oversight from Mutant Enemy Productions. When the Region 1 DVDs were released, Joss Whedon decided that they should not have the widescreen version, because the show was directed and filmed for a 4:3 ratio. The extra material in the widescreen versions is never important for the plot, although it gives additional views of the sets and backgrounds.