This encoder/decoder is a Javavascript version so shows translations in real time.
ROT13 is a common encoding message used to hide messages - it simply rotates the alphabet round by 13 postitions. For example, A becomes N, B becomes O etc etc. ROT-13 is not a secure method of communication as it's so simple to break, however it's a great way to hide text from a casual glance.
ROT 13 also goes by the name Caesar Cipher - as it was used by Julius Caesar in the 1st century BC! Much more can be read about this encryption method on the wikipedia page - https://en.wikipedia.org/wiki/ROT13.
For a more complete tool, visit my Encoder / Decoder page, which offers frequency analysis, and many other code breaking tools!