A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow), and can only occur during a full moon.
Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place, due to the smaller size of the moon's shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions.
The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth’s atmosphere into the shadow cone. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color. An alternative way of considering the problem is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth. Such a total eclipse of the moon is sometimes referred to as a blood moon.
Below are some views of the "blood moon", taken from the NWS La Crosse office (photos: NWS Meteorologist Stu Ireland).