See http://en.wikipedia.org/wiki/Dodecahedron
A dodecahedron is any polyhedron with twelve faces, but usually a regular dodecahedron is meant: a Platonic solid composed of twelve regular pentagonal faces, with three meeting at each vertex. It has twenty (20) vertices and thirty (30) edges. Its dual polyhedron is the icosahedron. To the ancient Greeks, the dodecahedron was a symbol of the universe. If one were to make every one of the Platonic solids with edges of the same length, the dodecahedron would be the largest.