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To understand what a penumbral eclipse is a little background information is needed. First, there are three types of eclipse's, penumbral lunar eclipse, partial lunar eclipse, and a total lunar eclipse. Passing through different parts of the earth's shadow causes these different types of eclipses. The two shadows are like over lapping cones with the penumbral cone being the larger cone. When the moon is passing through the penumbral cone, some of the suns rays still reach the moons surface. The penumbral lunar eclipse's are very difficult to observe even with a telescope and are typically only an academic interest.
Eclipses only occur during a full moon, but not all full moons are part of an eclipse. This is because the moon is tipped slightly, just 5 degrees, out of the plane of the earths orbit around the sun. Since the moon is tipped the majority of the time, it passes above or below earth's shadow. There are only 2 to 4 eclipses a year and they are one of the three discussed earlier. Thirty-five percent of the eclipses are penumbral eclipses.
Sources:
http://www.mr eclipse.com/Special/LEprimer.h tml
http://www.space.com/spacew atch/060310_night_sky.html

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