A
Caesar shift cipher is a
monoalphabetic substitution cipher where the
cipher alphabet is merely a shifting of the
plain alphabet.
An example of an english cipher alphabet with a Caesar shift of 1:
Plain 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
Cipher 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 A
This type of cipher is named after Julius Caesar because the first documented use of it (or any substitution cipher) for military purposes was by him.
ROT13 is a type of Caesar shift cipher with a Caesar shift of 13.