What are "sidebars" and "letterboxing"?

Sidebars
When a standard-definition program is viewed on a wide-screen HDTV, the HDTV may center the image on the screen with black, gray or white margins (known as sidebars) on either side in order to retain the 4:3 aspect ratio.

Letterboxing
In a similar fashion, when wide-screen (16:9 aspect ratio) content is transmitted to a 4:3 TV screen, the content provider may add black margins on the top and bottom of the screen to ensure that the entire width of the picture is visible. This is known as "letterboxing."