Adobe Photoshop has a number of functions that duplicate traditional darkroom printing techniques. Among these are tools for "dodging" and "burning". In darkroom printing, a photographer can selectively brighten or darken specific areas of the image by using special tools during the time the photographic paper is exposed under the enlarger. A small disk on a thin stick can be moved about under the light to reduce the exposure of certain areas, making them lighter, while a card with a hole cut in it can be used to increase the exposure of an area, making it darker.
Photoshop too has Dodge and Burn tools, and they are used for the same purpose, to alter the exposure of specific areas of the image. Of course, this being Photoshop, the tools are a bit more sophisticated than a bit of card and a stick.
Dodging and burning can be used for subtle adjustments to shadows and highlights, and is very useful for retouching portraits, or it can be used as a creative tool in its own right to make fairly radical alterations to the tone and atmosphere of an image. For example it can turn this rather drab shot...
I've included a full-sized version of the "before" image so you can try this tutorial for yourself. Click on the image to download the original.