High key and low key describe two opposite lighting and tonal approaches to a photograph. High key images are dominated by bright tones, light backgrounds, and minimal shadow, producing a clean, airy, optimistic feel. Low key images are dominated by dark tones and shadow with only small areas of bright highlight, producing drama, mystery, and weight.
The crucial point is that both are deliberate styles, not accidental over or underexposure. A high key portrait is lit and exposed on purpose to sit bright on the tonal scale while keeping detail, and a low key portrait is built around shadow on purpose while protecting its few highlights. Understanding exposure is what lets you push an image to either extreme without simply clipping it.
High key lighting uses soft, even, abundant light: a bright background, large diffused sources, and fill to open up the shadows so almost nothing goes dark. The look is common in beauty, fashion, newborn, and product work. On the histogram the data piles toward the right, and photographers often expose to the right deliberately while watching that the brightest areas do not become blown highlights with no detail.
Low key lighting does the opposite, typically using a single hard light such as a key light against a dark background, with the light carefully controlled so it falls on the subject and little else. Shadows are allowed to go deep and black, a rim light often separates the subject from the background, and the histogram sits heavily to the left. The style suits moody portraits, still life, and dramatic portrait lighting.
Choosing between them is a storytelling decision. High key feels light, gentle, and clean, while low key feels serious, intimate, or tense. Many photographers learn to control both because the ability to place the overall tone where the mood demands, rather than always aiming for an average exposure, is a mark of real lighting skill.
Building each look comes down to ratios. For high key, keep the background a stop or two brighter than the subject, use large soft sources, and add fill so the shadow side is almost as bright as the lit side, a near one-to-one ratio that flattens shadow. For low key, do the opposite: one main light, feathered and flagged so its spill does not reach the background, little or no fill, and a background several stops darker so it falls to black. In both cases meter for the tones you want to protect, the brightest detail in high key and the few highlights in low key, and accept that the rest of the frame will sit where the style demands.