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A lunar eclipse happens when the sun, Earth and moon line up, in that order. The moon is only ever lighted by reflecting the light of the sun back to observers on Earth. A few times a year, when the moon's orbit brings it directly behind the Earth, that light is blocked to one extent or another and the moon appears to vanish into the night for as much as several hours.
Penumbral Lunar Eclipse
What the full moon actually vanishes into is the Earth's shadow, formally known by its Latin name, "umbra." Because the sun is bigger than the Earth, the umbra is shaped like a cone, pointing out toward space. There's another cone going the other way, a partial shadow called the "penumbra." When the moon passes through this softer shadow, it undergoes what is known as a penumbral eclipse, but it's difficult to see and rarely noted other than by astronomers. Roughly one-third of eclipses fall into this category.
Partial Lunar Eclipse
A little less than one-third of the time, though most of the full-moon disk may pass through the penumbra, part of it passes visibly into the Earth's umbra. If even a little edge of the disk gets fully hidden, it's considered a partial eclipse. For instance, the lunar eclipse of 2009, on New Year's Eve, got only a tiny edge into the umbra, but that lasted for a full hour. The following eclipse, in June 2010, was also partial, but more than half of the moon was obscured, and the eclipse lasted for nearly three hours.
Total Lunar Eclipse
In December 2010, the lunar eclipse was a total one that lasted for almost five hours. The time in the umbra was only an hour and a quarter of that total. During a total eclipse, the disk of the full moon is usually still identifiable, even to the naked eye. Some of the sun's rays are bent around by the Earth's atmosphere, and the more dust there is in the atmosphere that night, the redder the moon will look.