Not really intended for security, just a mild amount of obscurity, ROT13 is a system often used for posting spoilers to books and movies, or off-color content that might offend some readers, on newsgroups and forums. It consists of text in which the letters of the standard 26-letter English alphabet are rotated 13 positions (wrapping around the end/beginning of the alphabet). Other characters are unchanged. It doesn't matter whether you rotate forward or backward, since 13 is exactly half of the alphabet and will wind up in the same place either way. This means that encrypting and decrypting are the same operation; doing it twice will leave the text exactly how it started. The result is text that looks "encrypted" and isn't obviously readable, but is easy to decrypt.
So, for instance, this sentence:
This is a sample sentence to be encoded in ROT13.
would be encoded as this:
Guvf vf n fnzcyr fragrapr gb or rapbqrq va EBG13.
Some geeky types who hang out a lot in forums where this is used get familiar with some common words, phrases, and acronyms as ROT13 encoded, such as "EBG13".
ROT13 is only well-suited for English-language text with few characters other than the ASCII letters, since anything else (accented letters, other alphabets, numbers, punctuation, mathematical symbols, etc.) are left alone.
Geeks have sometimes jokingly referred to something being encrypted in "super-secure" Double-ROT13 encryption; of course, something that's ROT13-encoded twice is actually just plaintext.