High Key is a term to describe images that are bright and contain little to no shadow. The style can also be described as bright, with overexposed backgrounds.
High key photography possess a lot of white and light tones. These images have very minimal mid-tones and blacks.
It is not devoid of blacks, it just has very minimal amounts of black in the image.
A good ratio to consider is 80% bright, 20% dark.
Low key is a term that describes images that are dark, and contain only a few highlighted/lit areas.
It is the art of lowering the exposure to create more shadows while creatively concealing and revealing details, which produces high contrast, dramatic images. A good ratio to consider is 25% light, 75% dark.