Three Point Lighting is a standard method of lighting used photography, video and film. It is a simple but versatile system which attempts to replicate the way natural light behaves.
Once you understand three point lighting you are well on the way to understanding all lighting.
The technique uses three lights called the key light, fill light and back light. Naturally you will need three lights to utilise the technique fully, but the principles are still important even if you only use one or two lights. as a rule...
1. If you have only one light it becomes the key (or main) light.
2. If you have two lights, one is the key and the other is either a fill or a backlight.
This is the main light. It is usually the strongest and has the most influence on the look of the scene. It is placed to one side of the camera/subject so that this side is well lit and the other side has some shadow.
This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will usually be softer and less bright than the key. To acheive this, you could move the light further away or use some spun. You might also want to set the fill light to more of a flood than the key.
The back light is placed behind the subject and lights it from the rear. Rather than providing direct lighting (like the key and fill), its purpose is to provide definition and subtle highlights around the subject's outlines. This helps separate the subject from the background and provide a three-dimensional look.
The Back Light is often also called the Rim Light or Hair Light.
The diagram below shows how setting the three lights at different heights, angles and intensity will model the subject and create the illusion of three dimensions.