1) Key Light - This light shines upon the subject and serves as its principal illuminator. More than anything else the colour, strength and angle of the key determines the shot's overall lighting design. This light is the strongest and the more important light.
2) Fill Light - This light also shines on the subject but from a side angle relative to the key and is often placed at a lower position than the key (about at the level of the subject's face). It balances the key by illuminating shaded surfaces, and lessoning or eliminating chiaroscuro effects, such as the shadow cast by a person's nose upon the rest of the face. This light is less stronger than the key light.
3) Back Light - This light shines on the subject but from behind, often to one side or the other. It gives the subject a rim of light, serving to separate the subject from the background and highlighting colours.
Here is our video demonstrating 3-point-lighting.

