I would link the site, but it is very unsafe for work and profane. Instead, here is an article by The Daily Dot chronicling its history on its 10th anniversary:
4chan is a primarily anonymous internet message board with fairly few rules and little moderation compared to other message boards. This makes it somewhat unique in world history, being possibly the only human society without a true social contract. While international, there are many more Americans on the site than members of any other country (source). While it is difficult to tell, considering the anonymity, it is also estimated that the majority of the site's users are young people. It is interesting that, in an environment like the internet where one could theoretically choose any identity they wanted, so many young Americans are choosing no identity at all. 4chan is not without a culture of its own, however - typically it is countercultural, originally founded for discussion of Japanese Animé (something still done on the site to this day) and now being a haven for various unusual political views and hobbies. The site is also unusually ephemeral in nature - "For every thread created on /b/, another was deleted. In other words, there was no archive." Though an archive has recently been implemented, the core nature is still that a discussion thread will only exist for a certain amount of time before deleting itself and being only viewable as a piece of history. These factors combined give 4chan a very recognisable collective identity, even though (and because) each individual anon has none. The unique internet slang means that individual expressions of American culture effectively disappear and all users, no matter the nationality, become anational; the ethos, however - complete and total freedom of speech - is perhaps the most American thing imaginable.
It is unclear what this means for American identity in the future. I am reminded of a quote I once read on the site itself: "In 30 years we could pass one another in the street and never know we spent every day of our youth together". This is the first time in history there has ever been such a situation, and it undeniably affects a large number of Americans. Certainly, 4chan's cultural affects will probably persist after the site disappears or becomes irrelevant, as it has given the nebulous "internet culture" many of its most well-recognised in-jokes and traditions (some of which have bled over into the mainstream and received coverage on news channels, one such incident involving the Occupy Wall Street pepper spray incident). Perhaps its users will grow to have families and friends like the average American, or perhaps they will remain alone, as so many 4chan users are. If this is the case we will have a sizeable demographic of American adults in the future who have no real analogue today, whose personalities were shaped by formative years spent on an anonymous countercultural website. It will be interesting to see.