"Rule 34" is an internet meme, perceives as an adage “Rules of the Internet", a list of protocols and conventions which first appeared on 4chan.
That in asserts that "if something exists, there is porn of it".
Rule 34 is said to come from a comic posted on the website Zoom-Out. There, an image with a caption "Rule #34 There is porn of it. No exceptions" was drawn by Peter Morley-Souter to depict his shock at seeing Calvin and Hobbes parody porn.
Then in May of 2009, Something Awful's user Electric Eggs posted a thread titled "Ask me about inventing Rule 34", in which he claimed to have created the comic with his sister after learning the about the adage in an IRC chatroom.
The name first appeared in the Urban Dictionary in 2006, submitted by user Nukeitall, who claimed the adage was commonly used on a variety of message boards.
Then On October 12th that year, the rule was included in the original "Rules of the Internet" list submitted to the Internet culture wiki Encyclopedia Dramatica.
When Rule 34 became a meme, due to the fact that porn is very popular on the web, the meme states that internet pornography exists concerning every conceivable topic. Since then, Rule 34 plays its role inside the ubiquity of internet pornography, especially among genres of fiction and hentai in which the characters engage in sexual behavior and/or activity.
In 2007, a Rule 34 database was created on Paheal.net with a searchable archive of Rule 34 images, with more similar websites appearing shortly after that.
Then in 2008, users at 4chan started posting numerous explicit imagery as parodies to illustrate the meme.
As the meme continued to spread and became increasingly popular, Rule 34 was listed by the Daily Telegraph as the "Top 10" internet rules and laws.
Since them the meme become part of mainstream culture, even beyond the internet.
The massive popularity of Rule 34, led to the creation on "Rule 35".
When Rule 34 depicts that there is porn for everything with no exception, Rule 35 follows by saying that "If no porn is found at the moment, it will be made." A variation includes: "The exception to Rule 34 is the citation of Rule 34."
Following that, is the "Rule 63", which expands the Rules of the Internet, by asserting that for every character, there is an opposite sex version of that character.
Rule 63 is often represented in a similar fashion to the Rule 34, but with users accompanying it with fan art that depict the gender-swapped characters engaging in sexual behavior and/or activity.
Among all of the Rules of the Internet, "Rule 34" is the most searched keywords, according to Google Trends.