Aspect ratio is simply the width to height ratio or proportion of a display system.
When you see high definition movies in a normal CRT (cathode ray tube) monitor or TV, you will be able to note that the top and bottom portions are blacked out. This is because the video which you are playing demands a better and a larger aspect ratio.
A normal CRT TV will have an aspect ratio of 4:3, whereas LCD TVs come in various aspect ratios, such as 16:9 or 16:10.These LCD TVs with high aspect ratios shows the complete video, occupying the full screen with no blacked out top and bottom portions.
So the next time you buy a monitor or a TV, look out for its aspect ratio.