What is the difference between a cliff and a dune?

A cliff is generally at the edge of a mountain or some other rock formation and is of hard rock, trees, roots, et al. It is formed by a sharp edge to a relative flat area and a long nearly vertical drop.

A dune is where the wind has blown sand or other small particles into small hills. They form attractive beach scenes along the coastline and also appear in sandy deserts. Dunes have no sharp edges nor long, nearly vertical drop-offs.