That's a Centaur, the upper stage of the Atlas 5 rocket that took the Lunar CRater Observing and Sensing Satellite (LCROSS) to our beloved Selene. It will impact against the Cabeus crater at 5,600mph, causing another crater about one third the size of a football field.
The Cabeus crater was selected because it's likely to show definitive proof of water in the moon, based on the information from the Lunar Reconnaissance Orbiter, Lunar Prospector, Chandrayaan-1, and JAXA's Kaguya spacecraft. The bombing is designed to measure the proportion of water—ice to dust ratio—in lunar regolith.
Watch http://www.nasa.gov/multimedia/nasatv/index.html, which will broadcast the event.
