Quantum key distribution (QKD) uses quantum mechanics to secure a communication channel. It enables two different parties to develop a shared random secret key known only to them, which can then be used to encrypt and decrypt messages.
What makes quantum key distribution unique is its ability to enable the two communicating users to detect unwanted attempts at trying to gain knowledge of the key. This ability is supported by a fundamental aspect of quantum mechanics: the process of measuring a quantum system in general disturbs the system. If an attacker is trying to eavesdrop on the key, the attacker must in some way measure it, thus introducing detectable anomalies.