Drawn porn. They've got all kinds of weird shit happening in and out. Tentacles, aliens, incest etc. In Japan, it simply means "pervert" (well, from what I've been told and read).
Really decent (save for yeah, those other weird types that utterly destroy your sanity if watched for long) as long as you don't start dreaming of those drawn girls, or you'll be digging your own grave.
It's interesting and in a way stimulating. I repeat again: As long as you realize that some weird shit there won't come true, the girls are JUST drawn, the body figures are out of rational proportion and not live in that "drawn" world it's all fine.
There are also dating sims out there and like previously mentioned, are sim-girl -related (Guy meets a lot of girls and screws them all then weird shit happens at the end). However, some do have stories. These games focus more on how to attain the girl in a focused manner without getting too much sex in between (I mean only at the end or if your "chosen path" is the girl you are pursuing; Even at the middle, you'll get laid. Easy shit, CAN be possible, but mostly impossible unless.. I don't know). Not only these sim-girled games focus on one girl, but focus on the whole story in general. All games (H-games) have different genres. Worth your time if you really don't have anything to do, or just want something else to meddle with.
All in all, hentai's good to a degree. Great alternative for real porn. Just realize none of those shit will happen in real life.
Guy 1: Want to watch hentai?
Guy 2: Er.. maybe some other time. I've got a date from our classroom.
Geek: Wow! that hentai is VERY VERY GOOOOOOOD. *drool*
Decent guy: Hey, don't tell me you're taking that drawn shit seriously.
Geek: What did you say?! Are you bashing the almighty hentai?!
Decent Guy: No. I have seen and enjoyed it. Maybe you need to get a fix pal. Those shit ain't real.
Geek: *sob* Why must you destroy my fantasies?!?!
Decent Guy: Whatever. They're good in their own way. Just not wholly worth fantsizing for in general however.