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From PC magazine click here : Digital Camera Basics: Understanding Exposure

Light is the essence of both film and digital photography. When light strikes film or an image sensor, it creates a photograph. Too much light, and the picture is “blown out” (too bright), with no details that you can discern. (See Figure 6-1a .) Too little light, and it’s so dark or muddy looking that you can’t really see anything. (See Figure 6-1b .) The trick is to get just the right amount of light to the photosensitive element (the film or image sensor) to record the scene accurately.

WOW, Click the link above to go to the PC Mag article. Lots of information on taking good photos. and lots of tech detail.