Hentai is a particular type of anime which is specifically about sex in any way, shape, or form. It may manifest itself in still picture, movie, or video game form. Most commonly it is just typical porn, but with anime characters. However, because it is animated, it sometimes can be even more sick, depraved, and downright revolting than any live action porn has ever been or ever will be. For example: One hentai movie may feature a man having sex with one or more women. That's just your basic porn. But some of the sickest hentai will tend to involve one or more of the following: Rape, Bestiality (sex with animals), Children (pedophelia), Tentacles (yes, like on an octopus), or pretty much anything sick you can think of. Depending on where you get your hentai, it can be perfectly acceptable porn, or a disgusting blemish on the face of humanity's creative tendencies. So beware. Tends to feature better-than-average-looking women with breasts that defy the laws of physics, strangely colored hair and, sometimes, unexplained animal ears. (These last two tend to crop up in all anime.)
1) Websites like fuckingtoons.com or hentaigamer.com.
2) Games like Maid's Story, Sexy Beach 2, or Secret Wives' Club (Hitozuma Hime Club in Japanese).