In frontal lighting, the light comes straight toward the form, so the planes get darker as they turn away.
Contre-jour lighting is the opposite: The head is seen against a field of light, which spills over the edges of the form.
The planes that are darker in one are lighter than the other.
Here are some charcoal studies using frontal lighting that I did as reference for a portrait of a spaceman on a paperback book cover (below, left).
I used contre jour lighting for the Asteroid Miner painting, right, which I did as a portfolio sample. I later sold the rights for a paperback cover, but the black type behind him sort of killed the effect.
Both frontal and contre jour lighting are a bit unusual, and can be used for dramatic or poetic effect.
Previously: Contre Jour Lighting
Asteroid Miner (without the type)