Aspect ratio is the relationship between the width and height of an image. With computers, you can think about aspect ratio as the relationship between the horizontal pixels and vertical pixels. Most screens are designed to display signals with an aspect ratio of 4:3, thus you see computers commonly supporting display resolutions of 800x600, 1024x768, 1280x960, and 1600x1200 (divide the width by the height and you find that all these resolutions have a ratio of 4:3). If you set your resolution to a ratio other than what your screen is designed to display, you will see that the image appears to be squished or pulled in one direction or another.
For example, my screen is set to a resolution of 1600x1200, therefore my monitor is set to an aspect ratio of 4:3 (1600 / 1200 = 4 / 3). If I set my resolution to 1600x900 (a 16:9 aspect ratio), my on-screen image will appear to be squished horizontally.
In summary, most screens are designed to handle images with an aspect ratio of 4:3, but some new screens use an aspect ratio of 16:9 which is called Widescreen format.
