Key light as the name suggests is the most important and the strongest light in a lighting scheme. In any kind of studio or artificial lighting a photographer needs to have a key light that is used to highlight the features of the subject. A key light is always a hard light which is concentrated and has a higher intensity than the rest of the lights in the frame. A key light is necessary to give a sense of direction to the frame.

If you see the image on your left, you can see that the directional key light is giving a direction as well as creating a mood in the frame. The character seems like a dark and shady one. You can see that the light is highlighting the edges, the nose and the entire shape of the dummy. This is the kind of effect a key light has in a light scheme.

A key light is always the first light to be placed in a scheme as this is the light that helps decide the rest of the scheme. Often put at an angle form 45 degree or 15 degree form the line of subject to camera. If you see the top view of the lighting scheme below, you can see that the key light and the fill light both have been put at an angle of 45 degree. This is because directional lights tend to bring out the texture and moods better. If just a key light is used in the scene then it results in high contrast ratio and unpleasant shadows just the way it is in the image with the dummy above.