At its most basic, photography is simply capturing light. The amount of light and the characteristics of the light determines how the photo will turn out. When you press the shutter release, the shutter opens allowing light to pour onto either the film or the image sensor. In this process, the camera uses three mechanisms to control the amount of light that is captured in a photograph: aperture, shutter speed, and ISO.
|My Canon 40 D|
|Wheel that adjusts aperture|
|Wheel that adjusts shutter speed|