What is a Crescent Moon?

Last night we went for a walk and I realized there is something I am totally clueless about... Science and astronomy.

William: "Look at the moon!"

Me: "Yup. That's a crescent moon."

William: "What is a crescent moon?"

Me: "Ummm...."

We continue to walk. I try to remember everything I can about science.

Matt: "Actually, buddy, it's the shadow of the Earth that makes it look like that."

Me: "Wait, is that for real?"

Matt: "Yes. Tell me you knew that."

Me: "Nope. I thought it was due to cloud cover."

True story. And I'm ok with it.