Aspect Ratio represents the horizontal width of a television or projection screen in relation to it its vertical height. For example, a traditional analog television has a screen aspect ratio of 4x3, which means that for every 4 units in horizontal screen width, there are 3 units of vertical screen height. HDTV's have a an aspect ratio of 16x9, which means that for every sixteen units in horizontal
In cinematic terms, these ratios are expressed in the following manner: 4x3 is referred to as a 1.33:1 aspect ratio (1.33 units of horizontal width against 1 unit of vertical height) and 16x9 is expressed as a 1.78:1 aspect ratio (1.78:1 units of horizontal width against 1 unit of vertical height).
For more details on Aspect Ratios and how they are implemented, read my article: Widescreen TV: The 16x9 Factor.