Three-point lighting
Consists of:
Key Light
Usually the major source of illumination
Often it's a hard beam of light that casts hard shadows
Typically establishes the DIRECTION which the light is coming
Fill Light
Generally on opposite side of the camera from Key and traditionally it's fairly close to the camera
Softer than key
"Fills" harsh shadows from Key
Backlight
Generally above and behind subject
Sometimes called a "hair light"
Helps separate subject from background
Often difficult to determine source of backlight
BUT WAAAAAIT...
This is a useful place to start, but not absolute formula
Always identify the key and fill (justify sources)
Ratio of key to fill has much to do with dramatic quality and intensity