Here is an explanation of the Caesar cipher.
A Caesar cipher (or Shift cipher) is a mono-alphabetic substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, the letter 'A' would be replaced by the letter 'D', 'B' would become 'E', and so on. The method is named after Julius Caesar, who used it to communicate with his generals.
a b c d e f g h i j k l m n o p q r s t u v w x y z -- will be
d e f g h i j k l m n o p q r s t u v w x y z a b c
A Caesar cipher script is available here.