Number look up

Full Sized Image

Shortly after sunrise or before sunset, when the moon is just a bright sliver, you can sometimes see the dark portion illuminated with a soft, dim glow. To photograph it you have to overexpose the lit portion, resulting in the loss of most detail there.

This dim light is sunlight that has first been reflected from the lit portion of the day side of earth, bounced of the part of the near side of the moon that is not directly lit, and then back down to your retina or camera. Cool, huh?

This diagram from Wikipedia demonstrates the concept very well. 

Diagram author attribution