Rule 34 is both a meme and a humorous truism that claims anything on the Web somehow has pornography attached to it somewhere. Whether it is an innocent picture of world continents, your garden in the back yard, or even aircraft, Rule 34 claims that somewhere there is a sexualized version available online.
About.com explains more about Rule 34 and its alleged origins here...
Well, seeing how the porn industry is pretty much solely responsible for the initial success of the Internet, I can see how “found porn” can be such a common occurrence. I’d say 70% of what people look up online at home (and 10% at work haha) is pornographic of some sort. We have a bunch of goofy examples in our humor section and funny pictures area.