They are ratios. If a picture is 2000 pixels wide and 1500 pixels tall, you divide both by 500 to get 4:3.
If it is 1800 pixels wide and 1200 pixels tall, you divide by 600 to get 3:2.
"A screenshot, sometimes referred to as a screencap or screengrab, is an image that shows the contents of a computer display. Screenshots let you capture exactly what you're seeing on your screen to share with others or reference later. Taking, saving, and sharing screenshots can be extremely...