Simply put, aspect ratio is the ratio of the width to the height of an image.
Many of my clients have gotten frustrated trying to decide what sizes to order when they're shopping for prints. If you're a photographer, I'm sure you've experience the same thing! Feel free to send this to folks in need of a visual.
1: Square images (1:1 ratio).
2. 8x10 and 16x20 images (4:5 or 8:10 ratio).
3. 5x7 images (5:7 ratio)
4. 4x6 and 8x12 and 20x30 images (2:3 or 4:6 ratio). I've found these to be the most popular sizes, as they don't crop out any of the image. They're full-frame.