Photography Jam explains some of the basic elements of cameras:
Put simply, a camera consists of a light-tight box that stores a light-sensitive device (either a film or a digital sensor), a lens that magnifies and focuses the image onto that light-sensitive device through a hole in the box (called the aperture) , and a shutter that opens and closes when you press the shutter release, exposing the film or sensor to the light; this is why a picture is sometimes called an exposure.
The amount of light entering the camera depends on the amount of light in the scenes that you’re photographing. A bright sunny cloudless day has more available light than a cloudy one, which in turn has more light than an indoor scene lit by tungsten lighting. To make the picture look right, we have to expose the film or sensor to the right amount of light. Too short, and the image will be dark, or underexposed..