The following tool allows you to encrypt a text with a simple offset algorithm - also known as Caesar cipher. If you are using 13 as the key, the result is similar to an rot13 encryption. If you use "guess" as the key, the algorithm tries to find the right key and decrypts the string by guessing. I also wrote a small article (with source) on how to crack caesar-cipher in an unknown context of an encrypted text.
If you want some in-depth knowledge, I highly recommend to read this book.
Travhf jvgubhg rqhpngvba vf yvxr fvyire va gur zvar.