There are two types of eclipses, a Solar eclipse, and a Lunar eclipse. Eclipses happen when the Moon and Earth fall into line with each other and the Sun. The orbital plane of the Moon is inclined at 5 degrees to that of the Earth, and so eclipses of the Sun and Moon can only occur at a New or Full Moon. These have to occur near to the crossing points of these two planes or "nodes," where the lunar and terrestrial orbits cross; otherwise we would have an eclipse at each New and Full Moon. The nodes are the places in the orbit where the plane of the Moon's orbit and the ecliptic intersect. Occasionally the Earth will move into a direct line between the Moon and the Sun, and stop all light from reaching the Moon. When this occurs we have a Lunar Eclipse, and the Moon is covered by a shadow. Lunar eclipses occur when the Moon passes into the Earth's shadow cone, which it can only do at Full Moon when it is in opposition to the Sun. A total eclipse can last up to 100 minutes. Lunar eclipses are more rare than Solar eclipses; a maximum of three will occur in a year, where the Sun can have as many as five. The last total Lunar Eclipse occured on December 20-21, 2010. Read more about this eclipse from NASA: The next total Lunar Eclipse will not occur until June 15, 2011 Read more about this total lunar eclipse from NASA: Occultation of the Sun by the Moon is called a solar eclipse. Solar eclipses occur only at a new Moon when the Moon comes between the Sun and Earth. The Moon blocks out the Sun's light on part of the Earth's surface and produces a Solar eclipse. The Sun is 400 times larger than the Moon, however, because of the proximity to the Earth, the Moon appears to be as large. If the Moon is far enough away to appear smaller than the Sun, then a special type of eclipse occurs called an Annular eclipse. This is when the dark Moon is surrounded by a bright ring of sunlight. The Earth and Moon both cast shadows in the sunlight. The shadow that the Moon casts upon the Earth has two regions. The main part of the shadow, which is directly in line with the Sun, is called the Umbra. When the Moon moves into the Earth's Umbra shadow, it blocks out all light from the Sun and a total Solar eclipse is observed. The other region of shadow is called the Penumbra. When part of the Moon is in the Penumbra, this causes a partial eclipse. The shadow this casts is not as dark as the Umbra, so it only blocks out part of the sunlight.
The next total Lunar Eclipse will not occur until June 15, 2011 Read more about this total lunar eclipse from NASA:
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The Moon in Modern History