Low Key.
Low key images are usually dark, predominantly black images. The most simple low key image is so simple. Sit someone down, light them using a table lamp, or a candle. There, low key portraiture.
I do it differently, I have used several lights in the past to produce a low key image.
Firstly.
I use this setup to produce skin texture Nudes. The light reflecting off of the skin produces a fabulous texture.
Alternatively.
The light above the camera is very low powered. The idea is to just lift the front,(facing the camera), side of the subject out of the deep black of the background. The lights either side from behind provide a rim lighting effect. In each case the background is black.
Thirdly, and possibly more conventionally.
Dark background, NONE of the lights are ona particularly high power. Each is set to seperate the subject from the background.