A lunar eclipse only happens around the time of a full moon, when the Sun, Earth and Moon are almost in a line (see diagram below). When the Moon enters the Earth's shadow, less light which reaches its surface has passed through the Earth's atmosphere and has been scattered by all of the dust and particles. The scattering removes most of the blue light leaving only redder colours, which is why the Moon appears orangey-brown.
During a Partial lunar eclipse, only part of the Moon is inside this shadow, but with a Total lunar eclipse, the Moon is entirely inside the Earth's Umbra (the dark cone-shaped shadow in the picture).
Because the Earth's shadow is much larger than the Moon, lunar eclipses can last nearly 4 hours, with a total lunar eclipse (Moon in the umbra) lasting up to 1 hour 40 mins.