There are a large number of image boards out there: Web-based message boards especially meant for people to post pictures. A lot of the image boards in the chan series focus on pictures based on anime and manga, with names like 2chan, 7chan, 420chan (known for its confrontations with white supremacist talk show host Hal Turner), and so on. Perhaps of the biggest and most popular of these is 4chan, an image and discussion board. Originally aimed at fans of anime and Japanese culture, it has expanded its focus to become something of a catch-all site.
4chan has separate message boards dedicated to various topics, most of them related to anime porn. Of particular note on 4chan is /b/, the "random" board. This is often the funniest and noisiest of all the boards, with a posting rate over ten times that of any other board. (According to Wikipedia, the posting rate on /b/ is over 100,000 messages per day!) In /b/ one will often encounter memes, or "inside jokes" - usually consisting of some phrase photoshopped onto a picture. Several popular memes have evolved on 4chan and become popular all over the Internet, including:
4chan was initially founded as an imitation of a Japanese image board called Futaba Channel, but it eventually surpassed the original in popularity (and raunchiness). While the majority of 4chan's boards still focus on anime and manga, /b/'s popularity has soared to the point where for most users, 4chan is /b/ and anything goes. (There's a reason why they're called "/b/tards!")
In a comparison between 4chan and alt.binaries.slack, I'd put it this way: 4chan has about 1,000 times the number of readers as alt.binaries.slack; 100 times the number of active posters; and an equal number of laughs. If you wade through the LAME and FAIL constantly taking place on /b/, in a short time you're guaranteed to come across something that will make you LOL or STFU and GTFO.
Much of the content of 4chan – especially the /b/ board – is quite raunchy and definitely not for viewing by minors. A lot of the content of /b/ is intended to be as offensive as possible, which is why there is an overabundance of tasteless pictures of gore, obscene hardcore porn fetishes, and repeated "nigger" references (which I personally skip over, as that stuff is rarely funny). If this crap overwhelms you, you might want to look at the /c/ (cute anime) and /co/ (comics) boards instead.
The key difference of 4chan compared to most other image/discussion boards is the anonymity of its users. The idea is that without a name attached, there's no kissing of the board moderator's ass, or not listening to someone because they've only been around for a week. Everyone is supposedly treated equally, though posters who choose to use names and especially ones who use tripcodes (a form of identification) are looked down on and many threads are derailed by people insulting them.
/b/ - the random board, has memes, porn, serious discussion, and things that make goatse look acceptable to show to your grandmother.
/a/ - for anime and manga, this is what the site was founded for.
/d/, /h/, /s/, /u/, /y/: the porn boards, with the exception of /s/, they're various forms of hentai. There used to be lolicon and shota boards, but were moved to a separate imageboard called notfourchan, since US law is very vague about such matters.
/v/ - video games and table top games (e.g. D&D). Has recently become a work safe /b/.
/g/ - technology, was formerly guro. Mostly computers, but it's great for discussions on alternative energy and Tesla.
/x/ - Breaking News, 4chan has a paranormal board. So far its standard paranormal fare. (psiwheels, lucid dreams, etc).
Note for minors: If the background of a 4chan page is blue, it's work safe.
For the lazy /b/-tards, here's a program written especially for you: Lurker. It's "A program designed to browse 4chan's /b/ and automatically save threads that would be of interest to the user."