The packing of spheres so as to occupy the minimum amount of space. In a single plane, each sphere is surrounded by six close neighbours in a hexagonal arrangement. The spheres in the second plane fit into depressions in the first layer, and so on. Each sphere has 12 other touching spheres. There are two types of close packing. In hexagonal close packing the spheres in the third layer are directly over those in the first, etc., and the arrangement of planes is ABAB…. In cubic close packing the spheres in the third layer occupy a different set of depressions than those in the first. The arrangement is ABCABC…. See also cubic crystal.
http://cst-www.nrl.navy.mil/lattice/struk/fcc.html An interactive version of cubic close-packing
http://cst-www.nrl.navy.mil/lattice/struk/a3.html An interactive version of hexagonal close packing