This is one of your three groups of the sexually explicit genre of anime/manga.

Hentai has a negative connotation to the Japanese and is commonly used to mean "sexually perverted". In slang, it is an insult meaning pervert or weirdo and isn't applied to pornography in Japan. 18-kin, adult manga, and AV are used to indicate pornography.

Hentai anime/manga is focused on sex, the 'yummy parts,' as some may say. It's heterosexual sex, and your homosexual sex is yaoi/yuri.

Cartoon porn featuring girls unrealisticly hot that none of us will ever have.

mostly what everyone else said. all i want is for someone to post an image. ^__^

A Japanese word denoting unfavorable (or favorable, depending on context and circumstances) sexual connotations.
A loose English translation would correspond to words like "perverse."
Contrary to popular belief (and to its usage in America), when used as a noun, this word denotes a person who fits this description and nothing more.

The honed science of painstakingly designing young Japanese girls with the greatest rapability ever imagined.

Hentai (hen-tie) is Japanese for "pervert" or "perversion". In America it is often used to refer to Japanese animated pornography.

Saying the first letter in hentai (H) with a Japanese accent sounds like ecchi (eh'ch-chee). This is a slang word used in Japan that carries the same meaning as "hentai", although in America it is often used to denote soft-core anime porn.

The difference between Hentai and Anime is simply this. Hentai is art of a Japanese drawing style that depicts sexually explicit scenes.

Anime is art of a Japanese drawing style not depicting sexually explicit scenes, though usually tells a story of fantasy or something that can't be done in real life.