Different examples of Lighting

High Key Lighting:

High key Lighting over Brad Pitt gives the picture an all around brighter look to a place that would be dark.

Low Key Lighting:

As we can see that only half of Brad Pitt's face is showing to add mystery or darkness to the character. In order to make this High Key Lighting a fill light would be needed in this case to make his entire face available for the audience to see.

Back Lighting:

The main source of light is coming directly from the back making everything in the foregroun appear dark.