Unlike most of the party games you've played before, Cards Against Humanity is as despicable and awkward as you and your friends.
-Cards Against Humanity (A Party Game for Horrible People)
Cards Against Humanity is a game. As one might expect from its name, it is a card game. If I had to tl;dr it in four words, it would be: "multiplayer competitive Mad Libs." That is, if it was Mad Libs written by 13-year-old male stoners. You can (quite legally) download the card set for free from the internet in PDF format and print them out yourselves - or you can purchase a high quality set from the makers. I recommend the latter, although the former is an excellent way to create your own expansion cards (and one that they themselves recommend).
The game consists of nothing but two decks of cards. One set is white cards with black text, and the other (smaller) set is black with white text. The white cards contain, on one side, a word or phrase. The black cards contain phrases with blanks inset for particular words - the game calls them 'questions'. As time goes on, 'expansion packs' have been released to keep current with the news cycle and to offer new and horrible phrases.
Gameplay is simple. One player each round is designated 'card czar.' That player doesn't play during that round; instead, they pick a black card from the deck and read out the question it contains. For example: "A romantic, candlelit dinner would be incomplete without ______." Each of the other players then selects from their hand of white cards one which has what they consider the 'best' option for that blank. They place their submission face down on the table. The card czar collects all the submissions and shuffles them, then (when all have been submitted) reads out each submitted answer. For best results, read the question 'with' each answer. Funny or apt voices are always a plus. The card czar selects their favorite answer, and the player who submitted that answer gets the question card to hold as an 'awesome point.'
Some cards require two or three different submissions (have two or three separate blanks). You can either submit them in stacks and rely on the card czar to order them, or submit them in ordered stacks and have an agreed-upon order of reading (top to bottom, etc.).
That's the basics of it. There are all manner of optional additional rules which can be found in the rules booklet (which can be read as part of the card download). But that's really it. Part of the game is deciding what sort of answer the particular player acting as card czar each round will appreciate most. The most apt you can make with your cards? The most random? The most filthy? The most Warholesque? You decide!
One thing I must point out - there are cards which are, inherently, awesome. That is, if you're the sort of person who plays this game while drinking/smoking weed and is out to garner the most laughs from your horribly depraved group of friends. I mean, how can "Pac-Man uncontrollably guzzling cum" or "firing a rifle into the air while balls deep in a squealing hog" not win? Seriously.
Best played with at least four, preferably 5+ people, while consuming your favorite party beverage/smokable. CARDS AGAINST HUMANITY.
Oh, and buy a set, if they're not presently sold out. They deserve it.