A low key photo is simply a photo that is “overall dark”. Like this one, of Serenity, made yesterday:
(Black backdrop, 200 ISO, 1/125 sec, f/8, softbox on left as main light, softbox on right as fill light, snoot behind right as hairlight, gridded gelled speedlight on left for red accent).
The nice thing is that the subject stands out because she is the only light thing; in particular, her eyes are.
A high key photo, you guessed it, is a photo whose overall brightness is high. Like this, of intern Daniel, also made yesterday:
There you have it.
Which histogram belongs to which photo?