Symmetric key cryptography is any cryptographic algorithm that is based on a shared key that is used to encrypt or decrypt text/cyphertext, in contrast to asymmetric key cryptography, where the encryption and decryption keys are different.
Symmetric encryption is generally more efficient than asymmetric encryption and therefore preferred when large amounts of data need to be exchanged.
Establishing the shared key is difficult using only symmetric encryption algorithms, so in many cases, asymmetric encryption is used to establish the shared key between two parties.
Examples of symmetric key cryptography include AES, DES, and 3DES. Key exchange protocols used to establish a shared encryption key include Diffie-Hellman (DH), elliptic curve (EC) and RSA.