Black Holes are truly amazing objects. They were first postulated by Albert Einstien as objects of infinite density. In laymens terms this means that the atoms cannot be squeezed together any further. Density of any object is basically determined by how close the atoms and sub-atomic particles are packed together. The closer the atoms, the denser the object. Iron is much more dense than concrete because the atoms of iron are packed closer together than the atoms of concrete.
In Black Holes we have an object in space of infinite density! How then is a Black hole formed? They are formed when very large stars die. And science is now pretty certain that there are billions of black holes in space; and perhaps one at the center of every galaxy. A Black hole forms when stars die. Stars that are eight to ten times more massive than our sun. Like our sun these stars are burning fuel. The fuel is formed by hydrogen atoms colliding with each other at such intense speeds that they fuse together and form helium atoms. As the hydrogen atoms collide, massive amounts of energy is generated. We feel this energy on earth as heat and see it as light.
However once the star runs out of hydrogen and helium to burn it collapses in on itself. If the star is very large, maybe eight to tens times larger and more massive than our sun a black hole is formed. All the mass, volume, atoms, size etc now become concentrated in one point called a singularity. It would be like squeezing the earth down to the size of a bowling ball.
The gravity or pull of a Black hole is so great that nothing can escape if it gets to close, not even light itself. We know that gravity on Jupiter is massive, because of the size of this huge planet. This was graphically illustrated when comet shoemaker levy's orbit carried it to close to Jupiter and the comet was pulled into Jupiter by the planets intense gravity and obliterated. So we can imagine the pull of gravity from an object millions of times more massive than Jupiter.
There are also super massive black holes. These are formed from stars that are from one million to several billion times more massive than our sun. The Andromeda Galaxy has a super massive black hole at it's center that is more than three billion times the mass of our sun.
Although we cannot see a black hole; we know they exist from the affect they have on other stars and objects that get to close to them. When we see stars spinning around an area at very high speeds; and that area appears like empty space, then we know something with a strong gravitational pull is causing this. We can thus conclude that the object acting upon these stars must be a black hole. If the stars or any object gets too close they are sucked in, forever becoming a part of one of the most intense and fascinating objects in science.