Hentai is most commonly used to mean cartoon or anime porn, at least outside of japan. I don't need to list all the different types like someone else seems to have done, because anything that happens in real porn, happens in hentai. It's as simple as that. If you can imagine anything eroitc, well it's probably already been imagined and made into a hentai by someone.
As for the literal meaning of hentai, it's actually not very commonly used as 'pervert' in japan or in anime, probably because it's so well known as also meaning cartoon porn. If you want to call someone a pervert in japanese, say sukebe
ok? Apart from that i think the other entries cover it quite well.
Oh, one more thing, like i said earlier anything you can imagine can be made into a hentai, and probably has, there are plenty out there where the girls have normal boobs and the guys have normal dicks as well. There are also hentai that have story lines as well, and quite good ones at that, unlike most normal porn, not everything is an excuse for sex sex and more sex. But dont confuse hentai with fan service
in an otherwise normal anime!
I was watching what i thought was a normal anime, and it suddenly turned into a complete hentai... not like i minded of course.