A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape it. They are formed from the collapse of massive stars, and their presence can be detected by their effect on nearby matter and the emission of radiation. Black holes come in different sizes, from small ones with the mass of a few suns to supermassive black holes with the mass of millions or billions of suns. They play a crucial role in the evolution of galaxies and the understanding of the laws of physics.