Aspect ratio is the ratio (proportion) between width and height of an image. It doesn't matter whether the image is measured in cm, inch or pixels. However, measurement in digital imaging depends on resolution (and this can be different for width and height) hence always pixel values are used to calculate the aspect ratio.
If an image is 200 pixels wide and 100 pixels high it has an aspect ratio of 2:1 as well as if it is 600 pixels wide and 300 pixels high.
Digital cameras often produce images with a 4:3 ratio, for example: 640×480 (VGA), 800×600, 1600×1200, 2048×1536 (3 megapixel), 2592×1944, 3264×2448, etc. The common frame size used with still photography on 35 mm film is approximately 36×24 mm, giving an aspect ratio of 3:2.