The full moon at perigee—the so-called "supermoon" of May 5, 2012.
Perigee is the point in the Moon's orbit which is closest to the Earth. What was unusual about this event was that the full moon happened to come at the same time as perigee. So this is the largest and brightest that the Moon appears.
The Moon's orbit around the Earth is elliptical. When this photo was taken, the distance to the Moon was about 221,800 miles. At its most distant point (apogee), the Moon is more than 30,000 miles farther away.