This is where the earth moves in between the sun and moon. The moon falls into earth's shadow. This is what you will see (also illustrated in the photo frames below):
The phases of a Lunar eclipse. 1st image http://www.wsanford.com 2nd image http://www.eclipse.org.uk
You will see a dark shadow gradually creep along the surface of the moon (just like when the moon creeps along the sun during a solar eclipse). You will realise instantly that it is the earth's shadow (unless you are ancient and think the moon's being eaten by a dragon or giant toad!) The shadow keeps creeping along, and just when you think the moon's going to disappear...... it turns RED!!
"How come?" you ask.
Well, you should know that light is made up of seven colours:- Red, Orange, Yellow, Green, Blue, Indigo and Violet. These colours make up what is known as the COLOUR SPECTRUM. The moon goes red during a lunar eclipse because the redder colours of the spectrum (the light source is the sun, just in case you weren't sure) are reflected off of our atmosphere, and these hit the moon, which then reflects the red light back to earth again. The point of totality is reached when the moon goes red.
A total lunar eclipse. Both images taken from http://www.demon.co.uk
Here is a diagram of how a lunar eclipse will occur:
The earth blocks the sunlight off from the moon. Diagram taken from http://www.eclipse.org.uk
Of course, you can have partial lunar eclipses when the moon passes through earth's penumbra, but this only dims the moon slightly. You could in fact not even notice it! Speaking of partial lunar eclipses, there's supposed to be one in July. The exact date is July 28, 1999, but don't get your hopes up. It's only supposed to be 40%, and as mentioned earlier, they are sometimes not noticeable. But have a look anyway just in case. Lunar eclipses last a lot longer than solar eclipses, probably because the earth's umbra and penumbra are a lot larger than the moon's. They can last for longer than an hour in some cases. One last thing, lunar eclipses can only take place at night (OBVIOUSLY) when there is a FULL MOON!