ROT-13 Encryptor & DecryptorRot13 or Rot-13 (short for rotate 13) is a simple letter substitution encryption scheme. It works by replacing the current english letters in a message with those that are 13 positions ahead in the alphabet. For example, the letter a is replaced by n, b by o, c by p, etc. Numbers and punctuation are not encoded.
You may find newsgroup postings that are rot-13 encoded. It is sometimes used to hide offensive material from accidental viewing (naughty jokes for example). Unless you choose to decode it, the message will just appear as a bunch of gibberish (such as "GBB ZNAL FRPERGF").
Rot-13 encoding can be used for other purposes as well. Jokes and discussions that give away the ending to a movie are sometimes encoded so that it cannot be read on accident. A major advantage of ROT13 over ROT(N) for other N is that it is self-inverse, so the same code can be used for encoding and decoding.