It was suggested this sticky be put under this title with a link to the Bars, Bars and More Bars post.
As an often asked question because once a widescreen TV is brought home, many people wonder why there are black bars top and bottom or on the left and right sides of the TV screen. These posts and essays delve into this subject.
Here is that link:
Bars, Bars and More Bars
An updated version is here:
Bars, Bars and More Bars
Also check out:
HDTV Pictures
DVD Pictures