Black holes are celestial bodies that are characterized by enormous mass and very little illumination. They are formed when very large stars die and collapse under the weight of their own gravity, which concentrates the mass of the star into a very small space, creating a powerful gravity well. A star with a mass of about 10 - 20 times the mass of our Sun may produce a black hole at the end of its life.
When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely collapses under its own weight. This can form a variety of objects depending on the size and make up of the original star. For a small star, they form a brown dwarf, (no light emitted), for a sun sized star they form a white dwarf, for a large star (10 solar masses) they form a neutron star and for much larger stars (30 solar masses or more) they form black holes, infinitely compressing back on themselves.
The gravity well surrounding a black hole is so great that light, or objects moving at the speed of light, are unable to escape at a point of proximity known as the 'event horizon'. To escape the gravitational pull you would have to go past the speeed of light, which is theoretically impossble. Objects or people who are unfortunate enough to enter the event horizon experience forces which are indescribable by conventional physics. There is no need to worry oneself about such complexities, however, as anything of substantial mass approaching said event horizon would first be subjected to terribly violent forces which are quite describable by conventional physics and are almost always fatal.
The Accreation Disk is the circle that forms around the event horizon. As matter is pulled in by the black hole, it slowly circles inwards towards the event horizon, before getting sucked in to some unknown fate.
Some of the more imaginative scientists have speculated that the mysterious event horizon of a black hole is a doorway to another fantastic universe, most simply agree that black holes really suck.