Reductio ad Hitlerum (a wordplay on "reductio ad absurdum") is a semi-humorous term coined in 1951 by Leo Strauss. It describes an attempt to invalidate someone's argument, position or opinion by making a connection to Hitler or the nazis. (In other words, that opinion or claim is undesirable or wrong because the nazis (at least allegedly) also held that opinion.) It can also be used as the end of a slippery slope argument (in other words, that the opinion or position, if allowed or accepted, would lead to something that the nazis did.)
Godwin's Law is a semi-humorous observation made in 1990 by Mike Godwin, which states that the longer a heated online discussion or flamewar continues, the more likely it is that somebody will bring up nazis into the conversation (most often in the form of accusing others of being like them.)
While both involve the concept of nazis, they are not really the same thing. The latter might involve the former, but not necessarily. (In the latter case, calling someone a nazi, or accusing them of exhibiting nazi-like mentality, is more an insult and personal attack, rather than a fallacious argument against what they are saying. In other words, the other person is accused of being like a nazi, rather than their argument being discredited or considered undesirable because of said argument having an alleged connection to nazis.) Legitimately bringing up Hitler, or the nazi party, into the conversation because it's relevant to the topic at hand is not generally considered a case of Godwin's Law. It usually only refers to unwarranted, unjustified, spurious instances, often used as an insult (or, in some cases, as a completely fallacious reductio ad Hitlerum argument.)
"Godwin's Law", however, seems to have completely superseded "reductio ad Hitlerum" as a concept, even in situations where the latter would be a more accurate description than the former. Whenever you see, for example, a YouTube video that's a critical response to someone else's video, and that someone else made a reductio ad Hitlerum argument about something, it's very likely that this response video will name "Godwin's Law", rather than "reductio ad Hitlerum". Most often this is technically incorrect, because an individual YouTube video is not an online flamewar, and the Hitler argument usually isn't brought up to insult somebody who the maker of the video is having a conversation with, but to discredit some position or claim. Exactly what reductio ad Hitlerum means.