A Symmetric Cipher is cipher that uses shared key (shared secret between the two parties.
Because the key is shared, the cipher is said to be symmetric in contrario to asymmetric where the secret is known from only one side.
The most known symmetric cipher is the Advanced Encryption Standard (AES)
Encryption algorithms which use the same and single key for both encryption and decryption are known as:
The symmetric cipher is quicker than an asymmetric one that's why they are generally used in a second phase of a SSL connection when the shared secret has been exchanged between the two parties via a connection encrypted by asymmetric cipher.
Symmetric Cipher are also used in data transition. For instance, you may send an email verification with a link that has an encrypted payload. If the user clicks on the link, you receive the data and you can verify that it was the data you send.
The single secret key must be shared and kept private by both:
To use a symmetric encryption scheme, the sender and receiver must securely share the key in advance. This is normally done after a connection is secured with an asymmetric cipher (because a symmetric cipher is less expensive in computational power). See section below asymmetric cipher.