A and S are the two common settings found on your camera mode dial. However, on a Canon camera; these two settings are AV and TV.
A(AV) stands for aperture or aperture value. On this setting you can adjust your aperture and the camera will set everything else to the appropriate setting. This used when the subject is still. E.g. landscapes.
S(TV) stands for shutter speed or time value. With this setting, you can adjust your shutter speed. The shutter on you camera is what physically controls the time of an exposure. This is the mechanical sound your camera makes, the shutter opening and closing (If it makes a sound at all.) This setting is used when your subject is moving. E.g. Sports, Animals etc.