Black holes are stars collapsed to the point that nothing, not even light can escape.
Escape Velocity
Understanding black holes first requires understanding the concept of escape velocity. When NASA launches the space shuttle it must have an initial speed of at least 11 km/s (25,000 miles/hr). If the shuttle's launch speed exceeds this speed it can escape Earth's gravitational field and make it into space. If the launch speed is less than this escape velocity, it will fall back to Earth.
The value of the escape velocity from a planet or star depends on the mass of the planet or star and its radius. During a star's lifetime, there are often times when the star changes its radius (size) without changing its mass. When this happens, its escape velocity changes. If a star is compressed to a smaller size without changing its mass, its escape velocity will increase. Basically the gravitational force on its surface is higher because it is more densely compressed. A greater speed is needed to escape the greater gravitational force.
Black Holes
A black hole is simply a star that has collapsed so much that its escape velocity is greater than the speed of light. According to Einstein's special theory of relativity, the speed of light is the ultimate speed limit in the universe. Light can travel at the speed of light, however anything with mass can not reach the speed of light. It can just come arbitrarily close. Nothing can travel faster than light. Hence when a star collapses to the point that its escape velocity exceeds the speed of light, nothing can escape, not even light. We have a black hole. Traveling into a black hole is the ultimate one way trip; there is no traveling back out.
Why would a star compress this much? At the end of their lives, the most massive stars explode as supernovae. If the central core of the star left after the explosion is at least about 2 to 3 times as massive as the Sun, there is no force known to modern science that can resist the inward tug of gravity. It will continue to compress until it collapses into a black hole.
Because no known force can stop the collapse, all the matter in what was once the star is compressed into a geometric point. It has a radius of zero! This point is called the singularity. The singularity has the same mass as the core of the star that collapsed into the black hole, compressed into a radius and volume of zero. Hence it has an infinite density. The distance from the singularity to where the escape velocity equals the speed of light is called the Schwarzschild radius or event horizon. The Schwarzschild radius of a black hole ten times as massive as the Sun is 30 kilometers. Schwarzschild predicted this effect from Einstein's general theory of relativity.
Although nothing can escape from inside the event horizon, black holes don't automatically slurp up everything nearby. It is possible to orbit a black hole without falling in.
