The Full Moon is the lunar phase where the entire face of the Moon is illuminated by the Sun.  In order for this to happen the Moon’s orbital position must be at the point where the Earth is exactly on a line between the Moon and the Sun.  Since the Moon’s rotational period, the lunar day, is almost exactly synchronized with its orbit, we on the Earth always see the same side of the Moon.  The USSR’s Luna-3 was the first spacecraft to send back images of the far side of the Moon.  The American Apollo astronauts also imaged the far side with high-resolution cameras.  These images show a heavily cratered terrain with few of the lunar maria we are accustomed to viewing on the visible near side.  One explanation for this is the possibility of a thicker crust on the far side that prevented molten lava from the interior rising to surface and flooding low areas.

¹http://en.wikipedia.org/wiki/Full_moon