A retroreflector is an optical element
which has the property
of reflecting any light ray
in the direction
where it comes from. That is, the reflected
ray is parallel
to the incident
There is two basic types of retroreflectors. Cat's eyes are composed of a converging lense followed by a mirror at its focal plane, perpendicular to the optical axis. If you carefully do the raytracing by hand, you'll see that this indeed a retroreflector. Rooftops (2D version) and cube corners (3D version) are simply plane mirrors that are perpendicular to each other. Again, if you care to do some raytracing, you'll see that those are retroreflectors, altough I didn't personally tried it in the case of a cube corner.
When you look into a retroreflector, you see only your face, your eyes, or pupils, depending on the distance from the retroreflector.