A black hole is a supergravitational  field that sucks  up other objects in space, nothing can escape from it's powerful pull...not even light. A black hole is created when a star 40x's the mass of our sun burns itself past hydrogen and other types of gas to  iron. When the star cannot support itself it will go supernova and all that is left is the black hole. When the black hole is created it never dies! The black holes are not picky eaters. They will happily eat gas, consume a star or planet. http://en.wikipedia.org/wiki/blac k_holes        

 This is an artists conclusion of a black hole.

http://en.wikipedia.org/wiki/Image:Black_Hole_Milkyway.jpg

There are different sizes of black holes. There are quasars. Quasars have electromagnetic fields of energy surrounding a young galaxy . Just like black holes they suck everything up. They are related to black holes the only diffrence is the electromagnetic field. These beasts first were theories but after the discovery of black holes the theory turned into sciense fact. http://en.wikipedia.org/wiki/Quasar 

There is also a supermassive black hole. Supermassive black holes are black holes that contain hundreds and billions xs the mass of our sun.  People don't really know that there is a supermassive  black hole in the middle of our galaxy 4,000,000xs the mass of our sun!!

There is another type of black hole called a stellar mass black hole. Stellar mass black holes have masses ranging from about 5-30 solar masses. These black holes are created by the crash of individual stars. Stars about 20 solar masses  may be destroyed and turned into a black hole. The cores of lighter stars from neutron stars  or white dwarf stars. 

http://en.wikipedia.org/wiki/Black_holes#Sizes

This is an artists conclusion of a quasar.

www2.keck.hawaii.edu/.../quasar_simonnet.jpg

If  you want to learn more about black holes check out this little video!!

http://spaceplace.nasa.gov/en/kids/live/shows/ep001/index.shtml