Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape. They form when large stars collapse at the end of their life cycles. There are four main types of black holes: stellar, intermediate, supermassive, and miniature. Stellar black holes are the most common, forming from collapsed stars, while supermassive black holes exist at the center of galaxies and can be billions of times the sun's mass. The concept of black holes was first proposed in the 18th century and later supported by Einstein's theory of general relativity, with the first black hole discovery in 1964.