There are different widescreen formats that can be used in a DVD or blu-ray.The most common is close to a ratio of 16:9 or 1.85:1
Difference between Wide and Full for DVD
It is possible to mix both widescreen and full screen on the same DVD on the same side.The problem is that most DVD players will not know when to change from widescreen to full screen.So if you leave the DVD player set to widescreen, any full screen content will look stretched side to side. The user can change the DVD player setting if this happens.Many DVD titles solve this problem in another way:
All Widescreen content is created normally to look correct for 16:9 ratio All 4:3 Full Screen content is reduced in size to look correct when played on a widescreen and encoded with black pillars on each side to fit.
Full Screen DVD
All Widescreen content is cropped on the sides and reduced to fit into a full screen playback using pan and scan. All 4:3 Full Screen content is created normally to look correct for 4:3 ratio.