When you take a picture the light meter within the camera determines how long the shutter should be open for and how wide the aperture should be, thus obtaining the correct exposure on the CCD. If the picture is too dark, it is underexposed, whereas if it goes the other way and is too light, it is overexposed.
Program Exposure - This is found on most digital cameras, programme exposure is an automatic setting by which the camera's metering system selects as appropriate aperture setting and shutter speed in an attempt to get the best exposure and performance out of the lens.
