A, S, M? What do they mean?

A stands for Aperture Priority Mode. This means that you set an aperture value on your camera, and the camera varies exposure length to make a good picture. Why would you want to set a constant aperture? Well, you need to do so if you want to control the depth of field, as in landscape photography or portrait photogrpahy.

S stands for Shutter Priority Mode. When you select "S", the camera will use a constant shutter speed and vary the aperture to achieve the correct exposure. This mode is useful if you want to achieve motion blur in your photos. You can choose a long exposure and the camera will handle the rest. For minimizing motion blur, such as in sports photography, you are better off using the camera's fast action mode.

M stands for Manual Mode. You have full control of shutter speed and aperture, and if your exposure comes out too light or dark, that's your problem. This mode is useful when your cmaera might have difficulty metering on exactly what you want such as scenes with wide variations in lightness and darkness. Also, lenses that are not compatible with your camera but still fit on the camera body can be used in this mode.