According to Robert M. Wald in his book 'General Relativity', a black hole is defined as "a region of spacetime from which nothing, not even light, can escape". Black holes may form when giant stars burn out and collapse in on themselves.

Black holes are similar to vacuum cleaners as they are cleaning up debris left behind in outer space by using the power of gravity to pull things towards them. The gravity of black holes is so powerful that even light can't escape, which is why they are black.

For more readings about black holes, check the following sites:
www.bbc.co.uk/science/space/universe/sights/black_holes/
www.damtp.cam.ac.uk/research/gr/public/bh_intro.html
en.wikipedia.org/wiki/Black_hole