Key light is directly behind the camera giving the subject an even exposure. This kind of lighting is most simple and easy to meter. In natural light conditions we can see this type of light in first hour after sunrise and last few hours of daylight.
Key light is coming from one side. Front lighting might render a subject 2-dimensional, side lighting gives it a volume and 3-dimensional look. This evokes stronger response from the audience especially when taking portraits.
Key light is behind the subject. Silhouette pictures are best examples of this kind of lighting. By adding fill light it can be used beautifully for portraits where back-light acts like a rim light.