The gravity at the center of a star is immense, constantly pulling the burning gases towards the center. The burning gases offset this gravitational pull, but when the star's nuclear fuel runs out, it explodes in what is known as a supernova. Much of the star is cast off into space; the rest can no longer resist the gravity at the center and is pulled inward. The gravity at the center is so strong that not even light can escape. This is a black hole.
A black hole only has two parts, neither of which is visible. The singularity is the point at the center of a black hole to which matter and light are attracted by gravitation. The event horizon is the other part of the black hole. It is an invisible border surrounding the singularity. The only way to locate the event horizon is to witness light or matter disappear as it makes contact with it.
The gravitational pull of a black hole does not extend far beyond its event horizon, so light and matter actually have to move toward the hole itself to be captured. The black hole's principal effect is to consume matter that strays near its event horizon. This matter is lost forever and is added to the singularity's density. A black hole created near another star may consume that star over time, growing as it pulls in the matter.
For many years it was believed that black holes remain in existence once created, but physicist Stephen Hawking proved that wrong in 1974. Using the laws of quantum mechanics, he ascertained that tiny particles of light and matter are created on a sub-atomic level within the black hole. These particles can sometimes escape, causing a slight glow in the event horizon. The hole therefore slowly evaporates, returning its absorbed energy to the universe.