Cryptography is the science that explore the mechanisms of hidden messages by key. There are two cryptography algorithms types: symmetric-key and public key.
Symmetric encryption is a type of encryption where only one key (a secret key) is used to both encrypt and decrypt electronic information. Also it has a symmetric channel to share the encrypted or decrypted messages.
The public key cryptography (or asymmetric encryption), uses public and private keys to encrypt and decrypt data. The keys are large numbers that have been paired together but are not identical. Only one key in the pair can be shared with everyone; it is called the public key.