The moon, like the planets and other moons, orbits in an ellipse. When it is close to the earth in that ellipse it is called perigee. When the perigee and a full moon coincide, the moon will be 14% larger than it normally appears. This interesting occurrence happens about every 18 years.
The perigee in March 1993 was four hours off of the full moon, pretty close. However, this year's will be only one hour from perfection. So get your best cameras ready!
Even though NASA chose the title of "Super Full Moon", I didn't like the connotation.
Their write up is here: http://science.nasa.gov/science-news/science-at-nasa/2011/16mar_supermoon/
NASA ScienceCast video