A histogram is a graph that shows how the tones of an image are distributed. Dark tones are on the left and bright tones are on the right. In an 8-bit image the scale goes from 0 (pure black) to 255 (pure white). The height of the graph at each point shows how many pixels have that brightness.
The shape of the histogram gives a quick idea of exposure and contrast. A graph pressed against the left or right edge shows clipping, which means lost detail in shadows or highlights. In editing programs, tools such as Levels and Curves let you change exposure and contrast while you watch the histogram change.
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