The aperture of a camera is a measure of how wide the opening is that lets light into the camera. In most point and shoot and SLR cameras this setting can be manually adjusted for every lens. Each setting is called an f-stop and the core scale is: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. Just like ISO each full stop allows twice as much light through the lens or half as much, again depending which way you go. Unlike ISO the larger the number the less light allowed through the lens.
On the above scale f/1.4 allows in the most light and f/22 the least. F/8 allows in half as much light as f/5.6.
Aperture controls not only how much light is allowed through a lens but also how much depth of field is in your picture. Depth of field is the amount of a picture that is in focus from the foreground (or front of a picture) through the middle ground to the background.
An aperture of f/22 allows more of the picture to be in focus than f/2 for example. F/2 is said to have a shallow depth of field (like the shallow end of a pool) while f/22 has a wide depth of field.
A shallow depth of field allows you to really focus attention on the subject of your picture because the background will be out of focus. A wide depth of field can be useful for landscapes where virtually everything is in focus.
In this first example you can see how the flower in the middle is in sharp focus while the flower closer to the camera in the lower left is out of focus and the background is a blur of other flowers.
As we increase our depth of field the foreground and middle ground flowers are becoming more clear in the photograph.
The foreground and middle ground are in good focus. Notice how the leaves in the background are clearer but not totally in focus.
Now at f/22 the background is in greatest focus possible for the lens.