Poe's law is an adage that states that, without a clear indicator of the author's intent, it is impossible to create a parody of extreme views so obviously exaggerated that it cannot be mistaken by some readers or viewers as a sincere expression of the parodied views.
The original statement of the adage, by Nathan Poe, was:
Without a winking smiley or other blatant display of humor, it is utterly impossible to parody a Creationist in such a way that someone won't mistake for the genuine article.
"Poe's law" was originally written by Nathan Poe in 2005, in a post on christianforums.com, an Internet forum about Christianity. The post was written in the context of a debate about creationism, where a previous poster had remarked to another user "Good thing you included the winky. Otherwise people might think you are serious." Poe then replied, "Without a winking smiley or other blatant display of humor, it is utterly impossible to parody a Creationist in such a way that someone won't mistake for the genuine article." The original statement of Poe's law referred specifically to creationism, but it has since been generalized to apply to any kind of fundamentalism or extremism.
In part, Poe was simply reiterating common advice about the need to clearly mark written sarcasm or parody (e.g. with a smiling or winking emoticon) to avoid confusion. As early as 1983, Jerry Schwarz, in a post on Usenet, wrote:
Avoid sarcasm and facetious remarks.
Without the voice inflection and body language of personal communication these are easily misinterpreted. A sideways smile, :-), has become widely accepted on the net as an indication that "I'm only kidding". If you submit a satiric item without this symbol, no matter how obvious the satire is to you, do not be surprised if people take it seriously.