A lunar eclipse occurs when the moon passes through Earth's shadow. Phil Plait has an excellent description of this upcoming event over at Bad Astronomy, including this helpful diagram:
|Note: objects are not to scale. Drawing by Shutterstock / fluidworkshop|
You can also see an explanation of how the eclipse will effect the Lunar Reconnaissance Orbiter (LRO), which is currently orbiting the moon, below:
When is the best viewing? The main event should take place between about 2- 5 am EDT. Plan accordingly.