According to Suetonius, Caesar simply replaced each letter in a message with the letter that is three places further down the alphabet. Cryptographers often think in terms of the plaintext alphabet as being the alphabet used to write the original message, and the ciphertext alphabet as being the letters that are substituted in place of the plain letters. When the plaintext alphabet is placed above the ciphertext alphabet, as shown below, it is clear to see that the ciphertext alphabet has been shifted by three places. Hence this form of substitution is often called the Caesar Shift Cipher. A cipher is the name given to any form of cryptographic substitution, in which each letter is replaced by another letter or symbol.
Type your message into the box labelled 'Plaintext', then click the button labelled 'Encipher Plaintext' to encrypt your message. You can produce new versions of the cipher by changing the size of the shift.