ROT13 is a simple substitution cipher

Substitution cipher

In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...

 used in online forums as a means of hiding spoiler

Spoiler (media)

Spoiler is slang for any element of any summary or description of any piece of fiction that reveals any plot element which will give away the outcome of a dramatic episode within the work of fiction, or the conclusion of the entire work. It can also be used to refer to any piece of information...

s, punchlines

Punch line

A punch line is the final part of a joke, comedy sketch, or profound statement, usually the word, sentence or exchange of sentences which is intended to be funny or to provoke laughter or thought from listeners...

, puzzle solutions, and offensive material

Profanity

Profanity is a show of disrespect, or a desecration or debasement of someone or something. Profanity can take the form of words, expressions, gestures, or other social behaviors that are socially constructed or interpreted as insulting, rude, vulgar, obscene, desecrating, or other forms.The...

s from the casual glance. ROT13 has been described as the "Usenet

Usenet

Usenet is a worldwide distributed Internet discussion system. It developed from the general purpose UUCP architecture of the same name.Duke University graduate students Tom Truscott and Jim Ellis conceived the idea in 1979 and it was established in 1980...

 equivalent of a magazine printing the answer to a quiz upside down

Transformation of text

Transformation of text is strategies to perform geometric transformations on text , particularly in systems that do not natively support transformation, such as HTML, seven-segment displays and plain text.-Implementation:...

". ROT13 is an example of the Caesar cipher

Caesar cipher

In cryptography, a Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number...

, developed in ancient Rome.

In the Basic Latin alphabet, ROT13 is its own inverse

Inverse function

In mathematics, an inverse function is a function that undoes another function: If an input x into the function ƒ produces an output y, then putting y into the inverse function g produces the output x, and vice versa. i.e., ƒ=y, and g=x...

; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding.