In a digital camera, there are three elements that work together to expose an image on the camera's digital sensor. Those elements are aperture, shutter speed, and ISO sensitivity.
Aperture refers to the size of the opening that lets light into the camera so that it can contact the image sensor. In most cases, the aperture is part of an optical system that works in conjunction with the shutter speed and ISO. The opening, or hole, can be adjusted by the camera settings, whether they are manually set or automatically set, to be very wide, very narrow, or somewhere in between.
The shutter button is pressed, and the aperture controls how much of an opening is allowed. The larger the opening the more light is allowed in. The amount of light is also controlled by the amount of time that the hole stays open. This is a function of shutter speed. Conversely, the smaller the hole, the less light is allowed to enter. Depending on the amount of available light around the subject of the photograph, the hole may have to remain open for a longer or shorter period in order to get a correct exposure for the picture.
You may have seen camera lenses referred to with something like "f/3.5". This is a reference to the largest aperture for that lens. F-stops are generally used to measure aperture in a digital camera.
The most confusing part about determining whether an aperture is large or small is related to the fact that larger numbers mean smaller apertures and smaller numbers mean larger apertures. Hence, a lens with an aperture of f/2.8 has a much larger hole than a lens with an aperture of f/9.0.
All cameras have aperture settings. The settings in a point and shoot camera are not used manually nearly as often as aperture settings in digital SLR cameras. Most point and shoot cameras do not have a very wide range of aperture settings available, so users tend to leave that setting on auto. However, if you are the owner of such a camera, you can find the manual control for aperture by checking the camera manual. You should experiment with changing the aperture just to see the effect it could have on your photography.
The effect of changing aperture values can be quite dramatic. There are times when having the entire photograph in perfect focus is desirable. Landscape photos usually fit into this situation. Landscape photographers will use a small aperture (remember, higher numbers indicate smaller holes, or apertures) such as f/13 or f/22.
But there are other types of pictures where it is not at all desirable to have the whole picture in focus. This is true of portraits. A good portrait has good focus on the eyes and face of the subject, but the area behind the person is quite blurry. This is a result of taking the picture with a very wide aperture such as f/2.8 or even f/1.4.
Another kind of photo that may look better when taken with a large aperture is a close-up, or macro, photo. Having a blurry background in a macro picture helps to draw the viewers eye to the intended subject. If background objects are in focus, it becomes quite distracting and draws attention away from the real subject of the photo.
Controlling the aperture is an excellent way to control the outcome of a picture. Knowing what will happen when you change the aperture setting on your camera will help you to become a better photographer.