Aspect ratio is the mathematical relationship between width and height.
For example, an aspect ratio of 1:1 is a square. Here are some
samples from Instagram.
The next aspect ratio is 2:3 (or 3:2). This is the aspect ratio
of DSLR cameras and 35mm film cameras. An aspect ratio can be
written as a single number, 1 number divided by the other.
For example, the 3:2 ratio of DSLR is 1.5:1
Finally, the 16:9 ratio is for video.