There are three main categories of hentai: 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 "bishonen," 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 (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, since the fandom for yuri consists almost entirely of males. The females in yuri are known as "bishojo," which, predictably, translates as "pretty girl." Shojo-ai ("girl love") is the female equivalent of shonen-ai.

The scope of hentai encompasses the entire range of sexual fetishes, including:

Ecchi, focusing on nudity, partial nudity, and provocative clothing rather than pure sex.

Catgirls and other semi-anthropomorphic characters, who display animal attributes such as ears, claws, and a tail. Generally, skin is made completely visible and not covered entirely by fur, a distinction from furries.

Bukkake, a common representation of a female being used to service as many males as physically possible, who then ejaculate on her. Often depicted in public or in areas with a large number of males present.

Tentacle sex has gained a notorious image in the United States, and is erroneously attributed to all of hentai in general. In fact, ordinary depictions of intercourse are more common in hentai; tentacles are more convenient for artists to draw in a situation that requires manipulation of the entire female body, which can be harder to coordinate in a situation with a group of males. Tentacles serve as the most basic phallic substitute, though it is usually avoided by those who prefer depictions of actual human intercourse.

BDSM, focusing on domination though use of ropes, tools, sex toys, and elaborate devices. Themes can include empowerment, restriction, and submitting to sexual urges.

Science Fiction, Fantasy and Horror, which can exist in its own setting, or be interwoven into a contemporary setting.

Incest, providing arousal through the taboo of interfamily relationships.

Futanari, a depiction of females who naturally have male genitalia, often exaggerated beyond normal proportions. Though debated whether attraction to such a depiction represents homosexuality in the viewer, heterosexual viewers have been known to enjoy futa. This may be due to the concept of male arousal and ejaculation being more familiar to the male viewer than female genitalia.

Lolicon, includes girls who are supposed to be under the age of consent in Japan (for sexual activities) [1], which is 13.

Guro, focusing on imaginative gore and mutilation. This may involve sexual violence and graphic scenes that are not typically represented in mainstream adult manga, and are a way for the artist to vent the most horrific images possible. T, Usually expresses thoughts of extreme gore and violence to its listeners.

Deformity represents subjects with altered physical proportions, multiple body parts, or artificial anatomy.

Scat and water sports, representing coprophilia and urolagnia.

Lactation or milk fetishism.

Maiesiophilia, sexual attraction to childbirth or pregnant women.

Shota, the depiction of boys having intercourse with other boys, men or women.