Aperture

Basic exposure is controlled by three factors: ISO, shutter speed, and aperture.  In this article, I'm going to discuss in very basic terms, and in my understanding, what the aperture is.  The aperture is the opening in the lens that allows light to pass through and hit the image sensor of your camera.  The size of the aperture is indicated by the f number (e.g. f/1.4, f/3.5).  The smaller the number the bigger the hole.  Note that increasing the aperture size by 1 stop will allow twice the amount of light to pass through.  For example an aperture f/5.6 will allow twice the amount of light to pass compared to f/4.

Below is a diagram to illustrate.  Take note that these are not the actual size of the aperture when set to the indicated stops, they're merely examples so that it is easier to understand the smaller the f number the bigger the hole is.

Some lenses have the same, or constant, aperture for all its zoom range while others have changing aperture sizes depending on the focal point selected (e.g. Canon 18-55mm).  Lenses with fixed aperture sizes are generally more expensive compared to their non-fixed counterparts.  The kit lens of the Canon 1000D is the Canon 18-55mm IS which has an aperture range of F3.5-5.6.  This means that at 18mm focal length, the lens has a maximum aperture size of 3.5 while at the longer 55mm end, the lens only has a maximum of 5.6.
 

Aside from controlling the amount of light that hits the image sensor on the camera, the aperture also controls the Depth Of Field (DOF).  DOF is the area of acceptable focus of an image.  When the lens is opened up (lowest f number), you will have a shallow DOF.  For example, if you're shooting a person's face at F/1.4, the eyes might be in focus but you will notice the image start to soften from the ears to the back.  When the lens is set to a bigger f number (smaller aperture), you will have a deeper DOF.  Which means, more of the image will be in acceptable focus.  When shooting portraits, you would want a smaller aperture so that you will have the background out of focus.  On the other hand, when you want to shoot landscapes, you want a smaller aperture so that you will have a deeper DOF and have the foreground, middle ground, and the background in focus (e.g. f/22).

Here is an example of a pictures shot with a large (small f number) aperture.  As you can see, the background is blurred.  This makes the subject of the image stand out  more. 

 

As I gain more experience, I may write more article about this and possibly include some starting/base aperture sizes that you can use for certain situations.

I'm not an expert in photography.  I am basically trying to share what I learned so that others like me might learn as well.  If you see some articles in error, or if you have more to share, please feel free to share/comment.