A bunch of bulletin boards on different topics under a single website. I think most of the boards are relatively SFW, on topics like gardening, origami, comic books, etc., but they also have a few distinctly NSFW boards such as the 'random' board, whose postings are mostly overrun by pornography, pictures of dead or dismembered humans or animals, and a lot of trolling for the sake of trolling. At least of late, they've gotten a lot of press for being one of the early sites (along with reddit, imgur, and a couple of others) on which the massive leak of stolen celebrity cell phone pics appeared, and then one of the places besides twitter that 'gamergate' seems to be happening. The main functional diferences from places like DU is that the vast majority of posts are 'anonymous', since posting doesn't require you to 'join' the site, and that (I think) only a limited number of posts stay available at a time, and older posts get shoved off into oblivion. I believe they were also the site where the Anonymous organization was spawned, as well as the 'brony' thing, which is something to do with guys who are really into My Little Pony.
I went over and looked around when they were in the news. Based on the postings, the membership ranges from folks who are fairly innocuous and there mainly to act like high schoolers to the truly mentally ill, who desperately need help, with a large helping of really angry folks. Obviusly this guy was on thr 'truly mentally ill' end of the spectrum.