4chan is a popular imageboard, which is basically like a message board with emphasis on images and usually humorous macros. People post topics about anything from anime to video games to pornography. Though, mostly...porn. Which is ironic; since the creator of the site, "moot," wasn't even 18 yet when he created it. In fact, he was actually 15. It's also home to the internet group known as Anonymous.
But to be truly honest, YouChewPoop shouldn't be insulting 4chan, as the majority of the YCP members are trying too hard to be just like it. Not to mention that almost everyone on YouChew likes at least one 4chan meme or something that became popular because of it.
4chan has had a lot of influence over pop culture, even going to the point of Rick Astley rick rolling various crowds during the 2008 Fosters Home for Imaginary Friends Thanksgiving Day Parade float. Yep, you heard me.
Not only that, but they brought us this masterpiece.
Aside from their various porn, one example of their various shenanigans is their various raids of the popular virtual world, Habbo. Various people, usually 4chan and additionally Ebaums World members, would dress their avatars as African-American people in business suits with afros and block the virtual pools claiming it was closed due to AIDS and stingrays (and occasionally "stingraids"). Other than the occasional mini-raids, on certain days there were extremely huge raids, with the various raiders crowding the room. Apparently, they were trying to be trolls by annoying the other players with their raids. Trying to make themselves look cool may have also been a goal. And it worked.
One of their recent raids happened on December 1, 2009; on World AIDS day. However, due to 4chan's 'No Invasions' rule, raids aren't allowed to be planned directly on the site. Not to mention that Habbo now has special scripts that ban people for making an avatar with a business suit, black skin, and an afro.
The Boards of 4chan
/a/ - Pictures and topics about anime and manga.
/b/ - The random board. Filled with nothing but nonsense, spambots, trolls, and porn! It is a constantly flooded because it is so popular. And since there are few rules, you can do almost anything. The afro blockers in the Habbo raids mentioned above would commonly make references to the board by speaking with / /'s around their b's. ("/B/ROTHERS, WE NEED /B/LACKUP")
/c/ - Cute anime is featured. Mostly involving bunnies, penguins, cats, and other things like that.
/d/ - Alternative hentai is featured; Tentacle rape, cat-girls, and other weird stuff.
/e/ - The ecchi section. Not necessarily hentai, but sometimes it borderline-hentai and people end up posting hentai anyway.
/f/ - Flash cartoons.
/g/ - Technology. Computers and stuff!
/gif/ - Animated gifs of porn.
/h/ - Hentai!! (what all true warriors strive for)
/hr/ - Hardcore porn.
/k/ - Weapons: Guns, knives, swords, etc.
/m/ - Mechas! Robots that are controlled by humans.
/mlp/ - My Little Pony: Friendship Is Magic. Made so bronies would finally have a board to keep all their pony-related content in. It worked.
/o/ - Cars and such.
/p/ - Photos
/r/ - Requests. Meaning...people asking for the sauce (source) of porn.
/s/ - Sexy, beautiful, women. Just pictures of ladies that are nice on the eyes.
/t/ - Torrents. "This is illegal, you know!"
/u/ - Yuri... Lesbian anime or manga. It's basically where males and lesbians go.
/v/ - Vidya is where people talk about games. But is filled with reaction images and pics that not related to vidyas, and has been compared to /b/ Jr.
/vg/ - Video Game Generals. /v/'s competent cousin where every topic is specifically for one game.
/vp/ - Pokemon. Made because /v/irgins sure do love their Pokemon, but now it's usually just furries and generation wars.
/w/ - Wallpapers of anime.
/wg/ - Just wallpapers....
/ic/ - The only place more barren, deserted than Venom.
/cm/ - It's like anime cute, only with males.
/y/ - Yaoi. Meaning... gay anime. It's where all the girls and gay guys are.
/an/ - Animals & nature. Nothing but nothing!
/cgl/ - Cosplay & EGL. People who like to dress up in strange costumes before Halloween.
/co/ - Cartoons and comics.
The Official Website (Warning: Many boards may be NSFW)
The Know Your Meme article (well, category) on 4chan
Chan Archive, where the supposedly best threads get stored.