When the Earth orbits into a position between the Sun and the Moon, the Earth's shadow (or "umbra") obscures the Moon.

This shadow can be divided into two parts

When the Moon enters the umbra wholly, a total lunar eclipse will occur; similarly, a partial lunar eclipse occurs when only part of the Moon is in the umbra. If the Moon does not enter the umbra but only the penumbra, the lunar eclipse in this situation is called a penumbral lunar eclipse (or "partial lunar eclipse").

When does a lunar eclipse take place?

Lunar eclipses take place when the Sun, the Earth and the Moon are aligned. Hence, this can only occur on the nights of full moon (i.e., on the fifteenth of the lunar calendar). However, as the Moon's orbit is not on the same plane as the Earth's orbit, the Moon does not lie in the Earth's shadow in most full moon situations. Similarly, this phenomenon explains why a solar eclipse does not occur at every new moon.

Is there such a thing as an annular lunar eclipse?

No. As the area of the Earth's umbra is much larger than the Moon, the Moon will be entirely covered by the Earth's umbra. Hence, the phenomenon of an annular lunar eclipse does not occur.