For low key images, they have to hit the darker half of the gray-scale and be predominantly dark. For high key it would be the opposite. predominantly shades from the lighter half of the gray-scale spectrum.  Loe key and high key tend to induce different emotions depending on how you use them. Low key might add mystery, drama, or eliteness to a picture while high key might add joyful, serenity.

Low Key

High Key