Top 4:3 Aspect Ratio, Below 16:9
Notice the 4:3 video edges have been cropped verses 16:9

Here a 4:3 picture has been stretched to 16:9 (Bad)

Here a 16:9 picture has been squeezed down to 4:3(Also Bad)

Here a 16:9 picture has been correctly converted down to 4:3 - This is called a "letterbox"(Good!)