Black holes are regions in space with gravity so strong that not even light can escape. The event horizon marks the boundary beyond which nothing returns, leading to a singularity where conventional physics breaks down. Stellar black holes form from collapsing massive stars and are scattered throughout the universe, while supermassive black holes reside at galaxy centers, including our Milky Way's Sagittarius A. Detection relies on observing effects like accretion disks and quasars since black holes themselves are invisible.