The existence of such objects was first suggested as far back as the late 1700s. However, it was Karl Schwarzschild (1873-1916), a German astronomer, who basically developed the modern idea for a black hole. Using Einstein's theory of general relativity, Schwarzschild discovered that matter compressed to a point (now known as a singularity) would be enclosed by a spherical region of space from which nothing could escape. The limit of this region is called the event horizon, a name which signifies that it is impossible to observe any event taking place inside it (since information is unable to get out).
For a non-rotating black hole, the radius of the event horizon is known as the Schwarzschild radius, and marks the point at which the escape velocity from the black hole equals the speed of light. In theory, any mass can be compressed sufficiently to form a black hole. The only requirement is that its physical size is less than the Schwarzschild radius. For example, our Sun would become a black hole if its mass was contained within a sphere about 2.5 km across.
Well inside the event horizon lies the heart of the black hole - the singularity. Everything within the event horizon is irreversibly drawn towards this point where the curvature of spacetime becomes infinite and gravity is infinitely strong. An interesting dilemma for astrophysicists is that the physical conditions near a singularity result in the complete breakdown of the laws of physics. Yet there is nothing in the theory of general relativity that stops isolated, or 'naked', singularities from existing. To avoid the situation where we could actually see this breakdown of physics occur, the cosmic censorship conjecture was proposed. This states that every singularity must have an event horizon which hides it from view - exactly what we find for black holes.
Alternatively, black holes can be classified by their two other properties of rotation and charge:
Observational evidence for black holes is, of course, not straightforward to obtain. Since radiation cannot escape the extreme gravitational pull of a black hole, we cannot detect them directly. Instead we infer their existence by observing high-energy phenomena such as X-ray emission and jets, and the motions of nearby objects in orbit around the hidden mass. An added complication is that similar phenomena are observed around less massive neutron stars and pulsars. Therefore, identification as a black hole requires astronomers to make an estimate of the mass of the object and its size. A black hole is confirmed if no other object or group of objects could be so massive and compact.