The Photoshoot: Three Point Lighting -

Three point lighting is a standard method used in visual media such as video, film and still photography. By using three separate positions, the photographer can illuminate the shots subject (such as a person) however desired, while also controlling (or eliminating entirely) the shading and shadows produced by direct lighting.

Key Light -
The key light, as the name suggests, shines directly upon the subject and serves as its principal illuminator; more than anything else, the strength, color and angle of the key determines the shot's overall lighting design.

Fill Light -
The fill 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 lessening or eliminating chiaroscuro effects, such as the shadow cast by a person's nose upon the rest of the face. It is usually softer and less bright than the key light (up to half), and more to a flood. Not using a fill at all can result in stark contrasts (due to shadows) across the subject's surface, depending upon the key light's harshness. 

The Back Light -
The back light (a.k.a. the hair, or shoulder light) shines on the subject from behind, often (but not necessarily) to one side or the other. It gives the subject a glimmer of light, serving to separate the subject from the background and highlighting contours.