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.
Different camera types have different aspect ratios. Full frame sensors, APS-C sensors and the smaller 1-inch sensors in many high-end compact cameras have the same 3:2 ratio as old 35mm film negatives. This gives a pleasing semi-wide aspect ratio for horizontal images, though doesn't work quite so well visually for vertical shots.