What is Three Point Lighting?

Actual definition:

The Three Point Lighting Technique is a standard method used in visual media such as video, film, still photography and computer-generated imagery. It is a simple but versatile system which forms the basis of most lighting. 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.

- Source: http://www.mediacollege.com/lighting/three-point/

The three lights involved in this technique all have different roles in the overall effect.

The Key Light is the strongest and therefor the main light, having the most influence on the photo. It is then placed at an angle (to the side) of a model to create some contrasts and shadows on the other side of their face/ body.

The Fill Light is on the opposite side of the Key light, making it the secondary light. By using this light source, the shadows that had previously been created by the Key Light's position alone will  be less intense or non existent. This light source is softer than the Key Light, and you can experiment with how far away this light from the subject of the photo.

The Back Light is placed at the back, and can highlight a hairline and give an overall professional look to the photo as it gives off a 3D effect. By having this light in place, the subject of the photo is more separated from the background.

If you take one of these lights away, the photo can look completely different. I, personally, am keen to experiment with the Key and Back light. This is because I like the dramatic effect of shadows and contours.

Source: https://www.youtube.com/watch?v=6p13FqFdgDc This video explains the effect of certain angles of light within Three Point Lighting.