This article explains Aspect Ratio

Aspect Ratio: Aspect ratio describes the ratio between the width and height of an image or screen. Common display aspect ratios are: 4:3, 5:3, 5:4, 16:9 The first number refers to the width, the second number refers to the height.

Example: A display monitor that is 20 inches wide x 15 inches tall has a calculated aspect ratio of 20:15. This number is reduced by dividing each number by the lowest common denominator, which in this example is 5, therefore, the specified aspect ratio of a 20 x 15 monitor would be 4:3

4:3 aspect ratio is considered a full-screen display or standard display it has a more box appearance.
16:9 aspect ratio is considered wide-screen display it has a more letter shaped appearance.