Basic Lighting Concepts
"Three Point Lighting" is the equivalent of the rule of thirds for lighting - it is the basic starting point for setting up lights on a subject. At it's most basic, it means you place lights at three points around the subject, with each light serving a specific purpose. The combined light from the three lights results in a nicely balanced scene:
The Key light is the main light source in your scene. It is generally placed in front and just off center of the subject (It will be somewhere to the right or left of your camera). Usually it will be at or slightly above your subject's eye level; if it is placed below eye level it can tend to produce odd looking shadows. It may be a soft or very directional light, but it will always be the brightest light in the scene.
However, by itself we end up with a very stark, shadowy image...which may work for certain subjects, but usually is not enough.
The second light in the typical three point lighting setup is the Fill light. Just as it's name suggests, it is used to fill in the shadows created by the Key light. Thus, the Fill light is usually placed at a 45 to 90 degree angle opposite the Key light (left of the camera if the Key is right, and vice-versa). The fill light may be placed high or low, and it will always be less bright than the Key.
With both the key and fill lights in place, we get a fairly nicely lit scene.
We could stop there, but one more light will help to strengthen our composition.
The back light is usually placed high behind the subject, at an angle nearly opposite to the key light. The aim here is to add a highlight along the edge of the subject's sillhouette, usually on the darker side of their body(the side with the fill light). This helps to define their shape more clearly and also makes them pop out of the background a little better. The Back light may be as bright as, or occasionally even brighter than, the Key light, particulary in situations where the subject needs to be separated from a busy background.
Because you are trying to get a hard edge with the back light you will usually use something directional such as a spot light. A back light may also be referred to as an Edge or Rim light.
Bring it all together, and you have classic three point lighting.
Low Key Lighting
In a low key composition, there is a large ratio between the key and fill light. This tends to produce strong shadows and contrast, and lends itself more to dramatic scenes.
High Key Lighting
High key lighting has a very low contrast ratio between key and fill lights. This produces a very smooth, low contrast, and generally bright scene. This is more typically seen in comedies and television sitcoms.