Well, I have seen a bunch of people talk about it, and I don't want to sound like a noob or anything, but what is it?
It's an image board, popularized by the "random" board, /b/. Also home to Anonymous, also famous for Project Chanology, Habbo raids, hacking, DDOSing, and most recently, this whole Jessica Slaughter thing and the 10 supreme pizzas and a box of wings (a $120 value!) they sent her after retrieving all of her info. They became even more famous after Fox had a special on them and called them terrorists.
Other boards on 4chan are pretty safe, /x/ is pretty neat, but /b/ is the internet hate machine.