A lunar eclipse happens at a Full Moon, when the Moon's tilted orbit brings it into the Earth's shadow, which can then be seen cast onto the Moon. While not as spectacular as a total solar eclipse, a lunar eclipse is much easier to see; and a total lunar eclipse is an amazing and beautiful sight.
This page attempts to explain how lunar eclipses work, and the different types of lunar eclipse.
A lunar eclipse is very different to a solar eclipse in terms of how the effects we see are created, because of our different point of view: in a solar eclipse, we stand at a particular point within the shadow of the Moon, and experience the effects of the shadow at that point; but in a lunar eclipse, we witness the whole of the Earth's shadow falling upon the Moon.
For that reason, the types of lunar eclipses don't correspond exactly to the types of solar eclipses. In addition, the Earth's shadow is much larger than the Moon's — because the Earth is larger — so it becomes possible for the whole Moon to be totally eclipsed, as this diagram shows (bear in mind that the scale is exaggerated; the Earth's shadow doesn't really cover a huge part of the Moon's orbit):
The shadow cast by the Earth has two parts:
As seen from the Earth, the penumbra and umbra form 2 concentric circles, through which the Moon passes during an eclipse. The type of eclipse seen depends on how close the Moon passes to the center of the shadow, as shown here:
The diagrams below illustrate how this works during the different types of a lunar eclipse. As you can see above, a total eclipse is always preceeded and followed by penumbral and partial stages; so the desriptions below of the penumbral and partial eclipse apply to these stages of a total eclipse, too.
In principle, a penumbral eclipse can be a partial penumbral eclipse (with only part of the Moon in the penumbra) or a total penumbral eclipse, where the entire Moon is in the penumbra; however, most penumbral eclipses are partial, since the penumbral shadow of the Earth is only about as wide as the Moon, so it's rare for the Moon to fit entirely within the penumbra without entering the umbra (and hence making a partial umbral eclipse). Once in a while, though, it happens — about 1.2% of all lunar eclipses are total penumbral eclipses.
Most penumbral eclipses are pretty uninteresting, since the Moon is
still quite brightly lit, except in the most advanced stages. Still, in a
deep penumbral eclipse, sharp-eyed observers should see a subtle
but distinct shading across the Moon at maximum eclipse. This will be
quite obvious in a total penumbral eclipse.
In the later stages of a partial eclipse, as the Moon darkens, red
coloration may become visible on the shadowed side of the Moon.
As with a solar eclipse, the distance
between the Earth and the Moon depends on the position of the Moon
within its elliptical orbit; however, due to the large size of the
Earth's umbra, the only effect of this is upon the size of the umbra
where the Moon passes through it, and therefore upon the duration of
the total eclipse.
Now go on to read about the interlocking cycles that dictate when eclipses occur, and why we don't get an eclipse at every Full Moon.