An app that will take plain text and encrypt it using a Caesar Cipher (also known as a shift cipher).
A Caesar cipher or shift cipher replaces each letter in the plain text with a letter N steps down the alphabet. For example, A becomes D, and B becomes E when N = 3. A famous example of this today can be found with ROT13 (rotation 13), where N = 13, meaning the alphabet is split evenly down the middle. When you reach the end of the alphabet the letters start over from the beginning.
Because the letters are only shifted up/down, there are only 25 keys possible, making this a cipher more for fun than security.
For more information: Caesar cipher @ Wikipedia