A transposition cipher changes the position of the letters in a message using a specific rule, called a key. The recipient, who also knows the key, reverses the process to get the original text. Writing a message backwards is a transposition cipher, although it is not a secure one, as it is relatively easy to break the code. Substitution ciphers replace each letter with another letter according to a rule or set of rules.
An example of early encryption is the Scytale.