In the simplest terms, a black hole is an object near which the gravity field is so intense that the escape velocity exceeds the velocity of light. Thus no light can escape from the black hole, hence the name.
To each mass corresponds a so-called Schwarzschild radius. If the mass in question is compressed to fit all in a sphere of this radius, the gravity at the surface will be such that no light can escape and we have a black hole. For an object of eight solar masses, this radius is 24km.
Such extreme compression is not known to occur anywhere except in sufficiently massive stars when they collapse under their own gravity after having exhausted all their nuclear fuel. See the article on white dwarves and supernovi for more on this process.
Black holes cannot be directly observed since no information escapes from them. Black holes can be seen through the effect they have on orbits of other bodies, for instance when a black hole is a part of a binary star system. If the black hole is in the proximity of other mass, this mass tends to fall in the black hole, forming a so-called accretion disc around the black hole. The extreme gravitational acceleration in the vicinity of the black hole causes matter to be tightly compressed as it spirals towards the black hole. This heats the matter to extreme temperatures and causes X ray and gamma ray emission. It is speculated that quasars be powered by black holes where matter which is heated to extreme temperatures as it falls towards the black hole radiates a large part of its mass away. Such reactions would be far more energetic than nuclear reactions inside regular stars. It is thought that supermassive black holes be found at the centers of galaxies.
General relativity predicts various anomalies around black holes. If an observer passed through the 'event horizon' of the black hole, i.e. approached within the Schwarzschild radius, space and time would reverse their roles for the observer. Movement in space would be one-way, towards the center, much like time moves one way outside the black hole whereas movement in time would be possible. As one approached the black hole, passage of time would slow down as compared to the passage of time for a more distant observer. The effect is similar to time dilation at speeds approaching the speed of light.
Electrically charged or rotating black holes have been predicted to open 'wormholes' through which an observer could pass to another point in space-time without traversing intervening space.
Where the matter and energy caught in a black hole go is unclear. The laws of physics break down inside the black hole. All matter falling to a point like singularity is inevitable once compression has proceeded past the Schwarzschild radius because movement in space becomes determined much just like the passage of time is determined in normal space-time. Compressing mass to an infinitely small volume would entail total entropy, i.e. complete homogeneity of the mass and complete loss of all information.
The Cassiopaea material states that a black hole is the grand scale physical reflection of the concept of service to self or of the 'thought center of non-being.' What is caught in black holes is recycled as 'primal matter' in the next cycle of the universe. It is as if the formerly trapped matter came out in a big-bang like phenomenon where the matter which no longer contained any information would then expand, cool and differentiate to form diverse elements, radiation etc. The analogy is apt but its literal truth is unverifiable. We could say that the condensed result of service to self development of one cycle becomes the inanimate matter with which the creative spirit plays during the next cycle.