An aspect ratio describes the shape of a film image or a display unit, such as a television or a cinema screen. Aspect ratios are often written as x:y - the x denotes the width of the image/screen, while the y represents the height.
Although the aspect ratio can be written in absolute terms, i.e. in inches for your television screen, it is common practice to simplify into a standard value. This can be achived by dividing the absolute x value by the absolute y value - we shall call this new number z. This form of the aspect ratio can be written as z:1 and, essentially, shows width per unit height of the shape and hence the bigger the z term is, the wider the image or screen is. Therefore, if the aspect ratio is 3:1, then for every inch in height, there are 3 inches across - or it can be said that the image/screen is three times as wide as it is high.
Widescreen can mean a variety of things, although it most commonly refers to a
picture size that is wider than the Image on TV's. On most TV's, the ratio of
the width to the height of the picture is 1.33:1. Widescreen can range from
1.66:1 all the way up to about 2.35:1. (see below).
The ratio all depends on how the director of the movie or TV show or video wants
it to be. The result from having a wider aspect ratio, is that there is more
information on the right and left sides of the picture.
Basically its the difference between these:
Most movies nowadays are shot in widescreen. I can't think of a movie that has been shown in a theatre with a ratio of less than 1.85:1, except for the Blair Witch Project and several Disney animated features. For the great majority of films, when the transfer is done to video, information is lost on the left and the right sides because the image is blown up so that the top and bottom match the top and bottom on the TV screen. In order to preserve the ratio that the director intended, the picture must be displayed with black bars at the top and bottom of the screen (this process is called letterboxing). While this is the way that it is meant to be seen, most people find the reduced screen size very annoying
This process of letterboxing reduces the overall resolution and detail of the useful image. Another less-used option that preserves as much detail as possible is anamorphic squeezing (which can be found on DVDs labelled with "Enhanced for widescreen TVs". All this leads to the advent of widescreen TV.
Ratio's you will have seen on your DVD's: