A lunar eclipse occurs when the Earth’s shadow passes across the full Moon, making it appear as though someone is rubbing it out slowly, or drawing a veil across it.
In order for a lunar eclipse to happen, the Sun, Earth and Moon must be aligned. One might be forgiven for thinking that this would happen every month, as the Moon orbits the Earth; however, the Moon's orbit has a slight wobble so most months it is either too high or too low to be caught in the Earth's shadow.
The Moon is still visible during a lunar eclipse. While in the shadow of the Earth, it has a browny red tinge to it. The name “blood moon” is often given to this appearance.
The most recent lunar eclipse occurred on 28 August 2007 and was visible throughout the Americas, East Asia and Australia. It was not visible in Europe or Africa as it was daytime in those areas at the time of the eclipse.
Elsewhere, of course, other moons are eclipsed by other planets.