Public-key cryptography is a cryptographic system that uses a pair of keys, a public key and a private key, to secure communication. The public key is available to everyone, while the private key is only known to the owner. The public key is used to encrypt messages, which can only be decrypted with the corresponding private key. This system allows secure communication between two parties without the need to exchange a secret key beforehand. Public-key cryptography is widely used in digital signatures, secure email, and secure web browsing.