Hentai is Japanese porn that is heavily imported into America. It is in animated format, which means that no one can be offended by its utter peverseness and that it can sidestep laws on women abuse (guro), child abuse (lolikon), and animal abuse (furry). It would be Stephen Colbert's favorite type of porn if he were the sort of person who looked at that sort of thing.
A short history of hentai. Nothing remotely graphic here. (although it does suffer from overuse of facts)