The aspect ratio of an image is the ratio of its width to its height. The larger the ratio, the ‘wider’ the image; the smaller the ratio, the ‘squarer’ the image.
4:3 ratio
This is an aspect ratio used by many medium format cameras, Micro Four Thirds cameras and compact cameras with small 1/2.3-inch sensors (like bridge cameras or point and shoot compacts). Visually, 4:3 images look slightly taller or less wide (however you want to look at it) with horizontal images, but the 4:3 ratio is quite pleasing for pictures taken vertically.