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In photography, exposure is controlled by a variety of things--the size of the lens opening, the film speed, and the duration the lens remains open taking the picture. In digital photo editing, we can adjust exposure further, with the Brightness and Contrast controls.
Brightness, as the name implies, is the amount of light in the picture. The longer the lens was open and the wider the lens aperture, the brighter the resulting picture will be. Every photo editing program will have a Brightness control. Changing the brightness setting will adjust the colors of the pictures as if the photo was taken with a wider or narrower aperture.
However, increasing the brightness can cause the picture to look washed out. This is where Contrast comes in. Contrast is the range of dark and light in the picture--the spectrum between the darkest and the brightest regions of the picture. Changing the contrast will make the brights brighter and the darks darker, which will counter-balance the changes made by the Brightness control. Brightness and contrast are generally used in tandem in most photo editing projects.
In most projects, it's rare to have a photo that needs overall brightness and contrast adjustments. What's more common is to have a picture that needs adjustments to small areas. For example, a dark cityscape against a bright blue sky, or a portrait with sunlight behind the subject, would likely be ruined by changing the overall brightness and contrast. These pictures need smaller, focused adjustments. In the old darkroom days, the only choice the photographer had was to dodge or burn. With modern photo editing programs, however, he can use a Lasso selection set, and then apply Burn, Dodge, Brightness, Contrast, or even Levels and Curves adjustments, to only those parts of the picture that really need it.
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