High key images, in simplest terms, are understood as pictures that are overall very bright and have “a lot of white in them”. While this is true, it doesn’t really refer to white only, but to the light and brightness in general. Typical high key image appears “overexposed” ie. very light, however, there should be no actual overexposure (such as a complete “white out” or loss of details in the highlights).
This lightness imparts a feeling of freshness, cleanliness, happiness and often – youthfulness. So it's not surprising that high-key portraits are used for “safe” fashion and magazine images as well as bridal and child photography. Most high and low key images are achieved under perfectly controlled, studio conditions, with high-key pictures typically employing a lot more light sources. Diffuse lights are mainly used for this purpose, in order to avoid harsh (or even all) shadows.
Which brings us to CONTRAST
In most high-key images, contrast is minimal. This is achieved by diffusing the lights, either via diffusers, beauty boxes or by bouncing them off the ceiling or walls rather than shining them directly on the subject. The ratio of opposing light sources is small or equal, which results in less difference in intensity of lighting on different areas, and often in these shots you will use a background that has little tonal difference from the subject, or typically white and light backgrounds. Lighting a studio scene for a high-key photoshoot can be quite involved, with great importance placed on illuminating the background correctly to achieve pure white without a white-out. White, however, is not the only option in this setting because hues (warm vs cool) impart their own mood, so with warm hues potentiating the feeling of romance, you may wish to go for a bright but off-white background, such as in the picture below. Once this is done, decisions on how to exactly illuminate the subject follow. For all this cleanliness and smoothness, the final images are of no use if they are flat and indistinct. So, high key images which cleverly employ either a strong colour in places, or directed hard lighting on a portion of a subject, will “pop” beautifully, adding a little bit of an “edge” whilst retaining all the essential mood components of a typical high key image.
Low key images are pretty much the direct opposite of high-key ones. Appearing “underexposed” or dark, without actual underexposure and loss of details in relevant areas, they use high contrast to illuminate contours of a subject in order to emphasise it’s form. This results in moody and edgy images, with lots of mystery and drama, and a strong hint of luxury so it's hardly surprising that low key approach is often used in luxury product photography such as expensive jewelry (especially diamonds) and high-end cars.
Low key also works well in portraiture, especially the one concerned with showing surface anatomy or that aims to tell a story.
The light illuminating the actual subject doesn’t necessarily need to be hard, ie, it doesn’t always have to result in harsh shadows, rather, it's the overall set up with dark or black backgrounds and selective application of few lights, that achieves this look.
Correspondingly, the number of lights needed for low-key photography is a less. It's not uncommon to set up a very good low-key image with under 3 lights, sometimes even one is enough, especially if it is the outline we are concerned about most, although background illumination is often useful to prevent the subject “merging” with the background. This is not to say that low-key photography is “easier”, in fact, it takes a lot of skill to achieve a perfect low-key shot. Some like to think in terms of ratios of opposing light sources, which are often as different as 1:4, but really, with the development of digital and ability to live preview the images, it is a matter of following the artistic vision and understanding the lighting techniques, which will result in a gradual build-up of light sources, until the perfect atmosphere is achieved.