During a lunar eclipse the earth passes directly between the sun and moon thereby casting a shadow onto the moon. However, light refracts as it passes through the earth's atmosphere. The longer wavelengths of light (red) refract more and so bend around the earth to flood it in red.
This photograph was taken from outside my flat in Dundee during the eclipse of 2006. The many dark patches are not volcanic craters but the basaltic lava plains (maria) that were formed after the eruption of ancient volcanoes. The mare directly beneath the 'a' of andrew is the Sea of Clouds. The crater of Tycho can be seen just below and to the right of this mare. The Seas of Tranquility and Serenity are the largest maria in the upper half of the moon.