There are 2 commonly used aspect ratios - 4:3 is the standard size (think of a TV screen) and 16:9 (a widescreen TV). An aspect ratio of 4:3 means that for every 4 units of width, you will have 3 units of height.
While you may never need to figure out aspect ratio on your own, here's a quick lesson in how to do so just in case. If you know the width of your image must be 200, how would you determine the correct height?
To calculate the aspect ratio in this example:
200 / 4 = 50 this is our 1 unit of width.
We know we need 3 units for our height
50 * 3 = 150
Giving us an image measuring 200 pixels by 150 pixels with an aspect ratio of 4:3.
Correct Aspect Ratio:
Distortion caused by incorrect Aspect Ratio: