A quantum corral is a nanoscale ring of atoms on top of a metal surface, made with an STM (scanning tunneling microscope). Usually the ring is made up of the same substance as the surface. (Copper, for example.)
The interesting thing about a quantum corral is that it creates a standing wave in the "electron sea" of the surface. With the proper STM settings, these standing waves can be viewed. Adsorbates inside the ring disrupt the standing wave, as electrons diffract (or scatter) off of the atom or molecule.
Actually, any particle on a surface will cause ripples in the electron sea, but it takes a structure like a corral or a quantum gun to make the effect impressive