A black hole is an object which is so compact, and whose gravitational pull is so strong, that not even light can escape from its interior. The concept of a black hole is very bizarre and one feels that physics ought to prevent such objects from forming. But no, the universe contains countless numbers of black holes, many weighing a few or few tens of solar masses, and others weighing millions to billions of solar masses. Black holes are implicated in a wide range of astrophysical phenomena, and often play a critical role. Gas accreting on a black hole emits electromagnetic radiation before falling in. Astronomers study this radiation to infer a variety of information. In a couple of favorable cases, we even have close-up pictures of black holes. Other black hole systems emit gravitational waves whose study has spawned an entire new field. This talk will describe the kinds of black holes that have been discovered in the universe, a few of their observational manifestations, and what has been learned.