Photography Composition: High Key & Low Key
When a photographs light is exaggerated to the bright end of the spectrum it is called "high key" photography. When it is slanted to the dark end of the spectrum it is called "low key" photography.
High key photography is often used to portray a delicate or feminine intention. A white on white photograph brings to mind the innocence of youth and the fabric of dreams.
Low key photographs on the other hand highlight the dark elements with a photograph. If high key photography is known as "white on white" then low key photography can be known as "black on black". There are varying degrees to which you can make a photograph a low key photo. If you were doing a portrait you could simply find the darkest room possible and highlight only the eyes.
Alternatively you can keep most of the body visible but very dimly lit. In both cases the overall feeling will be much more mysterious than a high key photograph. In low key photography you will evoke feelings of mystery, night and secrets. In low key photography silhouette and shape prevail over color and light. Below is an example of a low key photograph.
Here is a video about this topic with respect to landscape photography.
video by Michael Shainblume
Here is a video with this topic in relation to cinematography. Most of this applies to still photography as well:
video by wolfcrow