It's an imageboard (a sort of forum where people upload images to go along with their posts) and it's pretty well-recommended you be 18 to go on. It hosts a ton of different boards for all sorts of discussion, like anime or sports or tabletop games etc. The boards that give it its reputation as "the toilet of the internet" as Fizzy so eloquently put it are /b/ and /v/. /v/ WAS their videogame board but now it's pretty much a continuation of /b/, which is their "anything" board. Basically The Cage but with more porn and everyone's meaner and dumber.
A way better website is 420chan, which started out as a collection of imageboards for drug users, but now covers a variety of topics that the 4chan boards don't, including their awesome Professional Wrestling board, /wooo/.
On both sites users post mostly anonymously, though you can choose to have a custom name, or a name and special identifier code, if you so desire.
That answer your question?