Diffie-Hellman Key Exchange is a method that allows two parties to securely share a secret key over a public channel. It relies on the mathematical properties of prime numbers and modular arithmetic. Each party generates a private key and a corresponding public key, which they exchange. The shared secret is then computed using their private key and the other party's public key. This technique ensures that even if an eavesdropper intercepts the public keys, they cannot easily derive the shared secret. Diffie-Hellman is widely used in various security protocols, including SSL/TLS for secure internet communications.