It's a..."community" (well, not really) with a bunch of boards. There's boards for all sorts of stuff, but most people know it for "/b/", the "random" board. That's where people post all sorts of overused stuff and research.
In the words of 4chan itself:
4chan is a simple image-based bulletin board where anyone can post comments and share images. 4chan's collaborative-community format is copied from one of the most popular forums in Japan, Futaba Channel. Different boards are dedicated to different topics, from Japanese anime, manga, and culture to videogames, music, and photography. Users do not need to register a username before participating in the community. Feel free to click on a board that interests you, and jump right in—anyone can contribute!
List of boards:
Japanese Culture
* Anime & Manga
* Anime/Cute
* Anime/Wallpapers
* Mecha
* Cosplay & EGL
* Cute/Male
* Flash
* Transportation
* Japan/General
Interests
* Video Games
* Comics & Cartoons
* Technology
* Television & Film
* Weapons
* Auto
* Animals & Nature
* Traditional Games
* Sports
Creative
* Oekaki
* Papercraft & Origami
* Photography
* Food & Cooking
* Artwork/Critique
* Wallpapers/General
* Music
* Fashion
* Toys
Adult (18+)
* lovey Beautiful Women
* Hardcore
* Hentai
* Ecchi
* Yuri
* Hentai/Alternative
* Yaoi
* Torrents
* Rapidshares
Other
* Travel
* Health & Fitness
* Paranormal
Misc. (18+)
* Random
* Animated GIF
* Request
* High Resolution
* ROBOT9000
EDIT: Yeah, Fjeld pretty much summed it up.
