A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the Sun, Earth, and Moon are aligned (in "syzygy") exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a full moon.
There are 3 types of lunar eclipses: A penumbral eclipse occurs when the Moon passes through the Earth's penumbra. A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. A total eclipse occurs when the Moon travels completely into the Earth's umbra.
For further information please see: https://en.wikipedia.org/wiki/Lunar_eclipse