This is a very simple substitution type cipher that was invented by Julius Caesar. This encryption algorithm translates each letter in the string a fixed number of positions in the alphabet. Then the encrypted message is decrypted by translating in the opposite direction. This algorithm worked well a long time ago, but it fails to conceal many important characterisitcs of the message.
This simple encryption method requires a key that shows how every letter in the alphabet is linked to a different letter. When encrypted the letters are replaced with their corresponding letter that they link with.