Black holes are the densest objects in the universe. The gravitational pull from these bodies is so great that no energy or particle can escape from the event horizon, the zone at which gravitational acceleration exceeds the speed of light. Given this information, it would be easy to assume that nothing can exit a black hole once it has crossed the event horizon, but it has been theorized that black holes may, in fact, radiate a special form of radiation known as Hawking radiation.
Empty space is not, in fact, empty. Even the most empty regions of space still hold minute fluctuations of energy within quantum fields. Like small ripples on a pond, these energy fluctuations can occasionally collide to form spots of higher energy. Within a quantum field, energy concentrated into a single point can manifest as a particle. When a quantum particle is created, an equal anti-particle must also be created. In normal space these opposing particles would collide almost immediately after creation, but certain forces at the event horizon of a black hole can prevent these particles from colliding.
When created at the event horizon of a black hole, one of the particles in the particle / anti-particle pair may be created within the event horizon while the other is created outside the event horizon. The particle within the event horizon is pulled inward to the singularity of the black hole while the remaining particle is allowed to leave the gravitational influence of the black hole. By separating the particles from each other, the black hole loses energy and, because of the famous E=mc^2, mass. Over time a black hole will “evaporate” and eventually disappear entirely.