Blackhole

http://www.shivaranjan.com/2006/02/17/simple-explanation-of-black-hole/

What Is a Black Hole?

Simply put, a black hole is a region of space that is so incredibly dense that not even light can escape from the surface. However, it is this fact that often leads to miss-understanding. Black holes, strictly speaking, don't have any greater gravitational reach than any other star of the same mass. If our Sun suddenly became a black hole of the same mass the rest of the objects, including Earth, would be unaffected gravitationally. The Earth would remain in its current orbit, as would the rest of the planets. (Of course other things would be affected, such as the amount of light and heat that Earth received. So we would still be in trouble, but we wouldn't get sucked into the black hole.)
There is a region of space surrounding the black hole from where light can not escape, hence the name. The boundary of this region is known as the event horizon, and it is defined as the point where the escape velocity from the gravitational field is equal to the speed of light. The calculation of the radial distance to this boundary can become quite complicated when the black hole is rotating and/or is charged.
For the simplest case (a non-rotating, charge neutral black hole), the entire mass of the black hole would be contained within the event horizon (a necessary requirement for all black holes). The event horizon radius (R_s) would then be defined as R_s = 2GM/c².

How Do Black Holes Form?

This is actually somewhat of a complex question, namely because there are different types of black holes. The most common type of black holes are known as stellar mass black holes as they are roughly up to a few times the mass of our Sun. These types of black holes are formed when large main sequence stars (10 - 15 times the mass of our Sun) run out of nuclear fuel in their cores. The result is a massive supernova explosion, leaving a black hole core behind where the star once existed.
The two other types of black holes are supermassive black holes -- black holes with masses millions or billions times the mass of the Sun -- and micro black holes -- black holes with extremely small masses, perhaps as small as 20 micrograms. In both cases the mechanisms for their creation is not entirely clear. Micro black holes exist in theory, but have not been directly detected. While supermassive black holes are found to exist in the cores of most galaxies.
While it is possible that supermassive black holes result from the merger of smaller, stellar mass black holes and other matter, it is possible that they form from the collapse of a single, extremely high mass star. However, no such star has ever been observed.
Meanwhile, micro black holes would be created during the collision of two very high energy particles. It is thought that this happens continuously in the upper atmosphere of Earth, and is likely to happen in particle physics experiments such as CERN. But no need to worry, we are not in danger.