Lighting Research

Three-point lighting
This type of lighting is a standard method used in visual media including videos, films, still photography and computer-generated imagery. Using three different positions, the photographer can brighten the object or person that is the main focus of the shot however desired, whilst also being in control of (or getting rid of) the shading and shadows that occur due to direct lighting.
The key light, as the name implies, shines directly upon the subject and serves as its main illuminator. More than anything else, the strength, colour and angle of the key are the main factors of the shot's overall lighting design.
For shots that are taken indoors, the key is usually a specialised lamp, or a camera flash. For shots that are taken outdoors during the daytime, the Sun itself can serve as the key light. In this case however, the photographer is unable to position the key light in the position he wants it to be in, so instead, he/she will arrange the shot to capture the sunlight in the most effective way possible, maybe after waiting for the Sun to position itself at the right angle.
The fill light also shines on the subject, but usually this is from a side angle relative to the key light and is often placed at a lower position (usually at the position of the subject's face). It balances the key by illuminating shaded surfaces, and reducing or getting rid of chiaroscuro effects, such as the shadow cast by a person's nose upon the rest of the face. It is usually softer and darker than the key light (up to half), and more to a flood. The absence of a fill can cause stark contrasts (because of shadows) across the subject's face, depending on the intensity of the key light's brightness. On some occasions, such as in low-key lighting, this effect is done on purpose, but shots that are intended to look more natural and less stylistic are shots that you would need a fill to produce.
In particular cases, a photographer may choose to use a reflector (it can be a piece of white cardstock mounted off-camera, or even a white painted wall) as a fill light instead of an actual lamp. Reflecting and redirecting the rays from the key light back upon the subject from a different angle can cause a softer, subtler effect than using another lamp.
The back light shines from behind the subject, often (but not necessarily) to one side or the other. This provides the subject with a rim of light, which is done to separate the subject from the background and highlighting contours.

Four-point lighting
The same lights as mentioned above are used, with the addition of an extra light known as the background light. This light is placed behind the subject(s) on a high grid, or quite low to the ground. Unlike the other three lights which are used to illuminates foreground elements like the actors and the props, the background light illuminates background elements, including walls or outdoor scenery. This technique can be used to effectively get rid of shadows cast by foreground elements onto the background, or to draw more attention to the background. It also helps to off-set the single eye nature of the camera, which means that it helps the camera give depth to the subject.