Depth of Field
As explained in my “Exposure in a nutshell" post, depth of field determines how much of your image is in focus. By adjusting the size of your aperture, you can increase or decrease the depth of field.
A large aperture like f/1.8 will isolate your subject and blur out the background. This is very good for portraits where you want the person to really stand out.
If you choose a small aperture like f/22, everything will be in focus. Great for landscapes when you want to see every detail clearly.