All digital cameras create an image by focusing light through a lens onto a sensor. When the perfect amount of light reaches the sensor, there is enough light to show details in the darkest areas, but not so much light that the lightest parts are too bright. The goal of a good exposure is to have details in both the lightest and the darkest parts of the image. Today’s cameras have fantastic built-in light meters, and letting the camera determine the exposure is very easy. » More: Understanding Exposure