1) A black hole is a region of space where gravity is so strong that nothing, not even light, can escape its pull. They are formed when a large star runs out of fuel and collapses in on itself due to gravity. 2) There are three main types of black holes: stellar black holes formed by collapsed stars, supermassive black holes found at the center of galaxies that are millions of times the mass of the sun, and theoretical tiny "pinpoint" black holes. 3) When a massive star runs out of nuclear fuel, its stability fails under its own intense gravity, causing its radius to shrink within its Schwarzschild radius and forming a black hole as its outer layers explode