The key is the brightest light and casts the primary shadows, giving a sense of direction to the lighting.
The main function of the fill light is to fill in the shadows produced by the key light without casting a distinct shadow of its own.
Backlights, sometimes called hair, rim or edge lights, are placed on the opposite side of the subject from the camera, high enough to be out of view.
A fourth basic light, the set light, may be used to illuminate the background. Usually the background is partly lit by the key and fill lights, as well as by several set lights.
Lighting choices should be made before beginning to film, and should be based on the lighting style you hope to achieve.