15-1 - 15-3

15-1
Key light: provides the main illumination typically mimicking an actual light source like a lamp or ceiling fixture.
Fill light: fills in shadows created by the key light.
Rim light: is typically behind subject and placed quite high
Background light: mimicks light that would naturally fall on the walls or other background like a lamp or fixture.

15-2
natural light is what would be there in a real life setting.

15-3
One way of background lighting is to enhance depth by high lighting surfaces like furnature in front of the background.