In the following pictures we started  with the back light shown on the left.   By scrolling up and down you can study the effect of other lights.

On the left we've used "straight-on" lighting similar to a flash on a still camera or a sun-gun mounted on a camcorder.

Even with back light there is little depth or dimension in the  features.  This harsh style of lighting should be avoided.

If we use only a key light and a back light (left) we achieve depth and dimension, but without a fill light there are dark shadow areas that hide detail.

On the right we have added a fill light to the scene above and arrived at traditional three-point lighting. 

Not only have form and dimension been preserved, but we've "opened up" the shadows to reveal detail.

The soft lighting on the left (with key and fill of equal intensity and of soft quality) reveals all detail, but leaves the subjects looking flat, without real form or dimension.