A black hole is a point is space where the gravitational pull is so strong that not even light can escape. It pulls nearby material and objects into it. Nothing can ever get out. A black hole isn't really a hole which is thought of as being empty. A black hole contains the most matter stuffed into the very small space. Because of so much matter in such a small space, the gravity is very strong.
One theory about the formation of a black hole is that this happens because of gravitational collapse. When a very heavy star, perhaps 10 times heavier than the sun, gets old, it is low on fuel and cannot keep its temperature high. This condition lowers the pressure pushing out from inside the star to less than its own gravitational force and it implodes. This is called a supernova. The material left over collapses in onto itself and forms a black hole. This type of black hole is called a stellar-mass black hole.
Over time the black hole can become bigger and bigger by pulling in light and material around itself.
The second type of black hole is the supermassive black hole. This type of black hole is much bigger so scientists don't really know how one is formed. The stars which form these supermassive black holes may be millions of times bigger than the Sun. Scientists do know that there is a supermassive black hole in the middle of the Milky Way, Earth's galaxy. They believe that every galaxy has one. Black holes slowly evaporate and return their energy to the universe.
Black holes 'gobble' everything in sight with their strong gravitational pull caused by all of the material inside it. Scientists can guess there is a black hole by seeing all kinds of matter moving in a certain direction and increasing in its speed. In December, 2011, astronomers discovered that the black hole in the middle of the Earth's galaxy was beginning to inhale a nearby cloud of dust and gas. They believed that the black hole would finish eating the cloud up in several years. When that does happen, the supermassive black hole will emit high energy x-ray radiation.
Black holes are made up of 3 main parts. The very outer layer of a black hole is called the Outer Event Horizon. Within the Outer Event Horizon, the gravity is not as strong. The middle layer of a black hole is the Inner Event Horizon. The gravity in this layer is too strong for any object to get back out. The center of a black hole is called the Singularity. This word means a squashed star. If you squash a star into the size of an atom, its gravity becomes more powerful as it gets smaller. The gravity in the Singularity is very strong.
The idea of the black hole was first suggested by two different scientists in the 18th century: John Michell and Pierre-Simon Laplace. In 1967, a physicist named John Archibald Wheeler came up with the term 'black hole'.
Scientists really want to learn more about black holes and other strange and massive objects in the Universe. A space telescope called XMM-Newton was launched to orbit the Earth in 1999. It observes high-energy x-rays. The matter near black holes gives off x-rays just before it gets swallowed. Scientists observe these x-rays to try and understand more about black holes.
In summary, a black hole is a point in space formed possibly by an old star which has imploded and carried down into itself the leftover matter. This matter has shrunk in size but retained its mass and therefore has strong gravity. It pulls in light and matter around it and its gravity becomes stronger.