TVs come in two aspect ratios, 4:3 (the old standard) and 16:9 (widesreen). The aspect ratio is the relationship of the width of the screen to the height. So for every 4 units the screen is wide, it is three units tall. The widescreen TV ratio is sometimes called 1.78 to 1; the result of dividing 16 by 9.
The widescreen aspect was established by the ATSC for televisions and for broadcast standards. The aspect ratio for TV broadcasts is exactly 16:9 (or less if the broadcaster chooses not to fill the screen). Widescreen movies, however, are not 16:9, they are a little wider and so will still leave some black space above and below the image, even on widescreen TVs. Special circuitry in most TVs though will allow the viewer to zoom or otherwise force the picture to fill the screen if they choose. When studios make standard format DVDs, they reformat and crop widescreen movie pictures to fit on 4:3 screens. Widescreen movies on DVD vary in their aspect ratio, but always fit best on a widescreen TV.
Most high-definition televisions are widescreen, although some standard aspect HDTVs were being manufactured.
When 4:3 broadcasts and DVDs are played on a 4:3 TV, the picture fills the screen. When they are displayed on a 16:9 screen, the images are either surrounded on the left and right by bars (windowboxing) or the image is stretched or zoomed to fill the screen.
When 16:9 images are displayed on a 16:9 TV, the picture fills the screen. However, widescreen DVD movies aren't exactly 16:9 in most cases. The format for a film varies depending upon the choices made by the director. So even on a widescreen TV some DVD movies will have bars above and below (letterboxing) the picture. Most TVs allow you to stretch or zoom the picture to fill the screen. :When 16:9 images are displayed on a 4:3 screen, the result is a letterboxed picture.