Light reflected from the Earth lights up the shadowed portion of the Moon. When the Moon reflects this light back to Earth, we see it as earthshine. For a more complete explanation of earthshine, see AstroBob's explanation at this link:

http://astrobob.areavoices.com/2013/05/12/earths-ghostly-light-touches-the-crescent-moon-tonight/

Image taken with a TV85 refractor with a 0.8x TV field flattener, Canon 50D, ISO 200, 0.8 seconds, IEQ30 tracking mount.