Digital cameras come with various modes that allow you to take more control of taking the picture. One mode which your camera may have is called Aperture Priority which is usually symbolised on the camera by an A or Av.
So what does controlling the aperture allow you to do, and why would you want to do it?
Aperture controls something known as Depth Of Field which is the area of the image that is in focus. The rule is that as the aperture number gets smaller, which means the hole is getting larger, there will be a smaller amount of the image in focus.
The following diagram is a fairly crude diagram which explains depth of field a little more.
The red line is the point where you are focussing the camera.
The grey area is the area of the scene in front of you that will appear to be in focus for a given aperture.
It should be noted that this is not an accurate diagram, it just demonstrates what happens when you change the aperture settings. It should also be noted that the change from in focus to out of focus is a gradual one.
So really, depth of field is the distance of that graduation from the point of focus.
Why is this useful?
Using a small depth of field is a great way to isolate the main object in the picture. Sports or wildlife photographers use this technique to blur out distracting backgrounds keeping the main focul point sharp. Using a wide aperture also allows a faster shutter speed to be used which again helps keep moving objects sharp.
Using a larger depth of field is good for landscape pictures, when you want the area close to you and the area far off in the distance to be in focus. Using a large depth of field means that you usually need to use longer shutter speeds which means that some form of support for the camera is required to keep the camera still which the shutter is open. You will rarely see a landscape photographer without a tripod.