4chan is many things to many people. Originally Faux News viewed 4chan as a hazardous "Leftist Internet Hate Machine" filled with domestic terrorists. To others, it is that site full of astroturfers, weeaboos, trolls, children, and pedophiles that churns out some of the internet's most viral content. It has also developed it's own subculture defined by trolling, sociopathy, far-right-wing politics, and anti-feminism. This subculture as of 2018 permeates much of the internet including many incel forums.
At its core, 4chan is an anonymous Japanimation-themed collection of image boards based on the popular Japanese site Futaba Channel. 4chan is proof positive that the United States should not have stopped with Hiroshima and Nagasaki. So, not surprisingly, most of the nsfw imageboards consist of hentai and shock images. 4chan is like the butt-plug of the Internet in that it holds all the shit inside the rectums so it doesn't spray out all over. When 4chan goes down, it's like the Internet has explosive diarrhea.
4chan got it's start from Something Awful. Moot and some other faggots needed a site that they could host their creepy anime. Moot also had an obsession with 2Channel so he decided to download Futabally and translate it to English, then host it on a webserver. He originally sent it to 20 of his goon friends and eventually everyone in SA's Anime Death Tentacle Rape Whorehouse and every other Something Awful board full of closet anime pedophiles (all of them) started to use 4chan and the Internet Hate Machine began.
Launched on October 1, 2003; the site quickly became popular, expanded, and now features boards dedicated to a wide variety of topics, from anime/manga to videogames, music, literature, fitness, politics, and sports. /b/ was the first board to be created, and is the one that receives the most traffic. It's an open secret that /b/ is modded by the feds.
Loss of founder
On January 2015, moot stepped down as the site's administrator citing stress from controversies such as Gamergate as the reason for his departure. On September 2015, moot announced that Hiroyuki Nishimura had purchased from him the ownership rights to 4chan, without disclosing the terms of the acquisition. In October 2016, it was reported that the site was facing financial difficulties that could lead to its closure or radical changes.
4chan is known for The Fappening, the leak of Sarah Palin's emails during the 2008 election, Project Chanology, Gamergate, Anonymous, The Alt-Right, child porn, terrorist threats, and a handful of murders/suicides.