There are two common TV screen shapes that most folks will recognize — the squarish shape of conventional TVs, and the widescreen shape of today's HDTVs. The term used to describe TV screen shape is "aspect ratio" — conventional TVs, and some small LCD HDTVs, have a 4:3 aspect ratio; widescreen HDTVs have a 16:9 ratio.
TV shows also typically have a 4:3 or 16:9 ratio. While most new HD programming is in 16:9, a significant amount of TV broadcasts are still sent in the conventional 4:3 ratio. And it's the difference in shape between those two ratios that can result in a "pillar boxed" picture — one with black columns standing to the left and right of the image — when you watch a conventional 4:3 program on your widescreen TV.
So we have below the translation of our Aspect Ratio descriptors and how they relate to the screen (or setting) best suited for viewing: