Quantum key distribution (QKD) uses quantum mechanics to guarantee secure communication. It enables two parties to produce a shared random secret key known only to them, which can then be used to encrypt and decrypt messages. The security of encryption that uses quantum key distribution relies on the foundation of quantum mechanics, in contrast to the traditional public key cryptography, which relies on the computational difficulty of certain mathematical functions and cannot provide any mathematical proof as to the actual complexity of reversing the one-way functions used. The history and future development of quantum cryptography are also briefly discussed.