Elements of Design | Tone

Value, or tone, refers to the use of light and dark, shade and highlight, in an artwork. Some people also refer the lightness and darkness in an artwork as tints(light) and shades(dark). Black-and-white photography depends entirely on value to define its subjects. Value is directly related to contrast. Value is the relative degree of lightness in the graphic work of art or painting.

[http://www.instantshift.com/2009/12/19/100-beautiful-examples-of-black-and-white-photography/]