The information revolution is based on what a physicist would call a classical view of information. Quantum effects, though long known, were regarded mainly as another noise source to be managed by classical error correction. But only twenty years after Shannon's landmark paper, Wiesner noticed that they could be used to do some intriguingly nonclassical things, such as making impossible-to-counterfeit banknotes or multiplexing two messages into an single optical transmission from which the receiver could receive either one at will but not both. After a slow start, quantum information has developed into the most natural foundation for the mathematical theory of communication, extending Shannon’s theory as Einstein’s extends Newton’s. We review quantum information theory, especially the uniquely strong and private kind of correlation known as entanglement. Aside from enabling new kinds of computation and communication, entanglement helps explain the origin of randomness, why the future is less certain than the past, and, paradoxically, the macroscopic world's superficially classical appearance, which allowed quantum laws to remain undiscovered until the 20th century.