The difference is that the symmetric keys use the same algorithms for encryption and decryption. An asymmetric algorithm uses a different key for encoding and another for decoding. Asymmetric ciphers permit the encryption key to be public, allowing anyone to use the key for encoding. The encryption key is called the public key and the decryption key the private or secret key. Computers generate most modern cryptographic algorithms with specialized hardware and software devices.
Simple cryptograms use two main types of ciphers, a substitution cipher or a transposition cipher.
Of this type there are also monoalphabetic and polyalphabetic ciphers. A monoalphabetic always uses the same letter of the alphabet for the ciphertext letter. To the right is a simple wheel showing the use of a monoalphabetic substitution. A polyalphabetic cipher means that different alphabets were used to encrypt the message. This is used for more complex encoding and makes the message harder to decode.