Black holes are regions in space that have so much mass, nearby objects and even light can't escape its gravitational pull. Black holes are thought to form from heavy stars. When these stars end their lives in a supernova, their cores collapse and gravity wins out over any other force that might be able to hold the star up. (http://design.lbl.gov/education/blackholes/, 1/15/06)
Black holes cannot be seen because there is no light coming out of them. Because of this scientists have to find different ways to identify black holes. How do you prove the existence of something that you can't see? There are actually many ways to see if black holes really exist in our universe. One method is to look for objects in our universe that have a lot of mass, but are very small. For example we can prove that an object in space called M87 exists by looking at the mass, which is three billion times more than our sun, but takes up a space no larger than our solar system.
There are some very neat theories about black holes. One of these is being able to see the back of your head just by looking into the black hole. This effect, called an Einstein ring, is caused by the intense gravity around a black hole. When you are near a black hole at certain distances the light that leaves from the back of your head will travel though space that is bent so much by gravity that it will enter your eyes.
Another theory is being able to destroy information using a black hole. By the laws of quantum mechanics you cannot really completely destroy information, but scientists believe the usual rules of quantum mechanics do not apply in a black hole.