Source : Free On-Line Dictionary of Computing

rot13
     
        /rot ther'teen/ [{Usenet}: from "rotate alphabet 13 places"],
        v. The simple Caesar-cypher encryption that replaces each
        English letter with the one 13 places forward or back along
        the alphabet, so that "The butler did it!" becomes "Gur ohgyre
        qvq vg!"  Most {Usenet} news reading and posting programs
        include a rot13 feature.  It is used to enclose the text in a
        sealed wrapper that the reader must choose to open - e.g. for
        posting things that might offend some readers, or {spoiler}s.
        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.
     
        [{Jargon File}]