Hentai (変態) is a Japanese word used outside of Japan, mostly in English-speaking and western countries, to refer to Japanese animation (anime), comic-book-style magazines (manga), and computer games ("bishōjo games", hentai games) with explicit sexual or pornographic artwork.
Unlike in the West, the line between hentai and mainstream works is somewhat blurred in Japan. Children anime can depict nude characters, for example in the Sailor Moon show the girls are nude during their transformation. Many artists add nudity as fanservice.
This form of Japanese culture acquired some popularity in the West thanks, to a large extent, to the Internet. Although there have been many pornographic comic books and animations produced in the West, they never were as popular as hentai is today.
Meaning of the word
In Japanese, the word hentai means metamorphosis, abnormality, or sexual perversion. It is never used to refer to "normal" sexual activity, or to adult manga or other entertainment that is merely sexually explicit. The terms 18-kin (18禁, literally "18-prohibited") meaning "prohibited to those not yet 18 years old", and seijin manga (成人漫画 "adult manga") are used.
Exactly how the term hentai came to refer to all sexually explicit content in American anime fandom is unknown. With the rise of the World Wide Web, however, the term was extensively promoted by pornographic sites selling access to (frequently bootlegged) erotic manga. Banner ads promoting these sites might, for instance advertise "live girls and hentai", with the latter meaning erotic manga as opposed to photographs. In addition, many people outside of anime and manga fandom had come to associate anime with a particular genre of extreme pornography (e.g., tentacle rape) which could easily be called hentai in Japanese as well.
"H" in Japan is now broadly used to refer to all sexual content or activity, so "H manga" are manga with sexual content—however, "H" and "hentai" are no longer interchangeable. Also, the term "ero" (エロ), short for "erotic", is now used more often instead of "H".
There are three kinds of anime porn: works that feature strictly heterosexual interactions (often abbreviated "het" by its users), yaoi, and yuri. Yaoi refers to homosexual male pairings, and yuri to lesbian pairings. Yaoi commonly features males of ambiguous gender in both physical appearance and mannerisms. These males are called "bishōnen", which literally means "pretty boy." The traditional "bear" of gay porn in other countries is very rare in Japan. The reason for the androgyny is that yaoi most commonly attributes its fandom to females, often teenage girls who tend to care more about the romance of the relationship than the actual sex. Yaoi also exists outside of the hentai genre, since it is a fairly ambiguous term that is usually applied to any form of anime that includes male homosexuality. However, it is different from shonen-ai|shōnen-ai (literally, "boy-love"), in which two males simply express romantic feelings for each other and never actually have sexual relations. As implied, yaoi, more often than not, involves sex or at least "fooling around" between the participants.
Yuri is very similar to yaoi, except that the focus is on homosexual female interactions, and the females in a typical yuri illustration or animation tend to be far less realistic than the males in yaoi, as is usually the case in most forms of porn. The females in yuri are known as "bishojo|bishōjo", which, predictably, translates as "pretty girl." Shōjo-ai ("girl love") is the female equivalent of shōnen-ai. Shōnen-ai is far more popular in Japan than the US.