The following explanation for Godwin’s Law appears in the Wikipedia:
“Godwin’s Law … is a mainstay of Internet culture, an adage formulated by Mike Godwin in 1990. It is particulary concerned with logical fallacies such as reductio ad Hitlerum, wherein an idea is unduly dismissed or rejected on ground of it being associated with persons generally considered “evil”.
The law states:
As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.”
Godwin observed that people had increasingly begun to compare anyone and anything they mildly disliked with Hitler and/or Fascism….
Although in one of its early forms Godwin’s Law referred specifically to Usenet newsgroup discussions, the law is now applied to any threaded online discussion: electronic mailing lists, message boards, chat rooms, and more recently blog comment threads and Wikipedia discussion pages….[emphasis added]
There is a tradition in many newsgroups and other Internet discussion forums that once such a comparison is made, the thread is finished and whoever mentioned the Nazis has automatically “lost” whatever debate was in progress. This principle is itself frequently (though perhaps incorrectly) referred to as Godwin’s Law.“
Essentially, Godwin’s Law is a variant of the ill logic of guilt by association or argumentation by ad hominem (personal insult), whereby an argument itself is not attacked through logic – rather, it is countered by attacking the person who expresses the argument, either directly by name-calling (ad hominem) or indirectly by claimed affiliation or comparison with some allegedly undesirable group (guilt by association).