Autofocus points are shown when looking through the viewfinder of most SLR and DSLRs, even a few point and shoot cameras use them. So what are they? They are designed to help you know where your camera is focusing and to let you fine where the camera focuses.

What Do Autofocus Points Look Like?
Autofocus points generally are shown as small squares when you look through your camera's viewfinder. Often they will have a set of brackets or a circle around the middle point.

When a camera uses one of these autofocus points (when you press the shutter button halfway down), the point(s) being used will light up briefly. Red is a favorite color of camera manufacturers to use for this purpose but some cameras may light up the autofocus point as green or another color.

What Do Autofocus Points Do?
Autofocus points as locations in your camera's field of vision where the camera will focus. This means that the camera will select one of the squares on your viewscreen to be the sharpest part of the photo. The number of possible autofocus points depends on your camera. Some cameras have a 9 point system (like the example image on this page), while other cameras have 11 points or even 45 points. If no part of your composition is sharply focused at one of these autofocus points, the camera will likely refuse to activate the shutter (this feature can be overridden in most cameras). This also means that if a large object is in your viewfinder your camera may try to insist this is your primary subject instead of your actual subject, resulting in a blurry image.

Can I Change the Autofocus Point?
There are a couple of ways to change the autofocus point. Some cameras have the option of manual autofocus point selection, in some cases you can even select a group of points. A menu setting in the camera should activate the feature if available and then arrow or thumb wheels are used to scroll through the points while shooting. Another way to change the autofocus point is to set your camera to always use the center autofocus point. Center your viewfinder on your subject and press the shutter button halfway to lock focus. Then, without removing your finger from the shutter button, recompose your shot. This method also helps ensure better light meter readings in most situations.