Start with an event horizon:

http://en.wikipedia.org/wiki/Event_horizon

In general relativity, anOutside the event horizon of black hole is another interesting boundary - the photon sphere:event horizonis a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible.

http://en.wikipedia.org/wiki/Photon_sphere

Aphoton sphereis a spherical region of space where gravity is strong enough that photons are forced to travel in orbits. The radius of the photon sphere, which is also the lower bound for any stable orbit, is:

which is one and half times the Schwarzschild radius.