Widescreen resolutions are resolutions that are of a 16:9 (more common) or 16:10 (slightly less common). The majority of computer monitors and TV's produced today are widescreen, and thus feature a native widescreen resolution. Widescreen resolutions can cause problems with older games that were built for a 4:3 screen, resulting in either a stretched screen or black bars of unused screen space. Most games today will feature built in support for widescreen resolutions, while some games may require tweaking to force such a resolution.
Common 16:9 resolutions include:
Common 16:10 resolutions include: