Back in the day, in 1990, to be specific, before UseNet was transformed into little more than a digital petri dish for porn spammers, Mike Godwin noticed  a disturbing pattern. In UseNet threads about guns and gun control, those who oppose gun control sooner or later reminded those in favor of gun control that Hitler outlawed personal ownership of firearms. In debates about birth control, those who argued in favor of abortion remaining a legal option were inevitably compared to mass-murderers in the context of Nazi death camps. And of course, in any discussion around censorship on the Internet, some sort of reference to Nazi book burning was predestined.

In response to what he perceived as a method of shutting down open discourse, Godwin devloped Godwin's Law of Nazi Analogies (generally referred to in shorthand as Godwin's Law):

Godwin deliberately "seeded" UseNet newsgroups and threads with his law, in one of the early examples of a deliberately crafted Internet meme. Fairly quickly, his law took on a life of its own and spawned several corollaries.

The point of Godwin's law is that in cases where the comparison to Hitler, or the Nazis, is an example of hyperbole, it is both in effective and actually detrimental, since making a trivial comparison between the current topic and the horrors of Nazi Germany, the deaths of millions, and the horros of the Holocaust is completely inappropriate and horrendously insulting to the memories of the those who suffered.

The official Godwin's Law UseNet Legends FAQ is here, and there's an interesting interview with Mike Godwin from the October 1994 issue of Wired magazine.