A few years ago, I've learned what Cards Against Humanity is on this very site, on these very forums, and I remember we had an amazing time playing it with other people from the community (and making custom decks which contained cards about the game and the community).
First, let's explain what the game is about for newcomers. Cards Against Humanity is a card game in which there are two kinds of cards: black and white. A black card contains a sentence with at least one blank to be filled (and a maximum of 3). It is visible to all players and changes every round. White cards are distributed privately to all players. They contain a word or a phrase which can be used to fill in the blank(s) (unless it's a blank white card, a feature of the online version, which we'll be playing, in which you get to type whatever you want as answer).
Every turn, one player is the Card Czar. The Card Czar does not play a card (or multiple cards, if multiple blanks) to fill in the gaps. Instead, he picks the white card (or combinations of those if many blanks) which he likes the most. Then, the player who came up with those white cards gets awarded a point (the Czar and other players don't know which white cards were played by who, that is only clear when somebody is selected by the Czar and wins the round).
Then, the black card is changed, players draw white cards to replace the ones they used, and the Czar is also changed (the position cycles between all players, it's not randomized). The game ends when somebody has amassed a set amount of points, which can be adjusted at will by the host of the game (whenever I talk about features, I mean the online version). The host can also adjust other things, such as the number of players allowed to join, the number of players allowed to spectate, the name of the game, the password of the game, the time after which people get timed out if they did nothing (you can (re)join mid-game, however) and the cards used.
The host can pick any of the many packs used in the original
https://cardsagainsthumanity.com , but also add ones from
https://cast.clrtd.com , where anyone can make their own decks. As the host, you can add those by typing /addcardcast and then the deck's unique 5-character code in the game chat. For instance, if you want to add the deck I made as a host, you type "/addcardcast X5WM5" and press enter. If you want to make a deck yourself, feel free, but I recommend playing the game enough first, looking at other decks to see how it's done, maintaining proper spelling, and not repeating the same things that are already in another deck you plan to use or the deck will probably turn out to be bad. Additionally, use "save as draft" very often since the website's servers aren't always too stable and you might lose all progress (learned this the hard way). When you're ready, use "save & publish", but you might need to wait a bunch of hours for your deck to be approved, even if it's a private one. Before that, you can't use it in games.
The website on which you can play the game is
https://xyzzy.clrtd.com/zy/game.jsp . I'll be hosting those at some point, preferably at a time which covers both the European and American time zones reasonably well, and I will post the link of the game and the password here for those who want to participate (and in global chat as well).