A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. They are formed when massive stars collapse under their own gravity at the end of their life cycle. The boundary surrounding a black hole is called the event horizon, beyond which nothing can return. Black holes can vary in size, with stellar black holes being a few times the mass of our Sun, while supermassive black holes can contain millions or even billions of solar masses. They play a crucial role in the evolution of galaxies and are a key area of research in astrophysics.