Aspect ratio describes the relationship between the width of an image and its height.  To fill a widescreen television screen a picture needs to have an aspect ratio of 16:9 (16 by 9).  This means that for every 16 centimetres of width there are 9 centimetres of height.  This is the aspect ratio in which many programmes are now shot as they are made for widescreen transmission.

To show the picture on a standard television with an aspect ratio of 4:3 the sides can be chopped off the widescreen picture.  The normal television screen is then filled.  This is sometimes called "centre cut-out".

For more on 14:9 and deep letterbox go to the next page: 

If it's important to see the whole widescreen picture on a 4:3 TV then broad bands of black can be added at the top and bottom of the picture.  This is often called deep letterbox.

A compromise is to show the picture in 14:9 format which means that only the very edges of the picture are missing and there's a very narrow band of black at the top and bottom.  This is sometimes called "shoot and protect".

For much more about aspect ratios and for a useful resource to print and read here is a link to the Widescreen Book.