Hentai is a Japanese word that can be used to mean "metamorphosis" or "abnormality." However, that word has developed a strong negative connotation with the Japanese, and has become commonly used to mean "sexually perverted."
In western countries, hentai has become used as slang for sexually explicit anime, manga, and computer games. In Japan, such sexually explicit material is generally referred to as one of the following: "ju hachi kin" (which means the material is prohibited for sale to persons under 18) and "ecchi/Hentai anime" (which refers to sexual and/or adult anime).
The English use of the word hentai has more in common with the Japanese word "ecchi," which is used to refer to any sexually explicit content or behavior. The origins of the Japanese word "ecchi" isn't entirely clear, but it is believed it could have been a shortened form of hentai, and used as a polite codeword in the 1960s. It is also not entirely clear how the term hentai came to be used in American anime fandom to refer to all sexually explicit content. It is guessed that when the World Wide Web rose to prominence, the term was extensively used by adult sites selling access to what would usually be bootlegged erotic manga.
When it comes to hentai, there are two main categories: works that feature primarily heterosexual interactions (usually referred to as "het") and works that feature primarily homosexual interactions (yaoi refers to male pairings, and yuri refers to lesbian pairings). Hentai anime allows elements of sexual fantasy to be represented in ways that would be impossible to film, which includes portrayals of sexual acts which are physically impossible, or are unacceptable in society.
The scope of hentai encompasses a wide range of sexual fetishes. Bakunyu depicits women with large breasts. BDSM utilizes domination through the use of ropes, tools, and other devices. There is also hentai anime that include bestiality, incest, exhibitionism, deformity, and even some that depict sex with tentacled creatures. Some hentai is labeled as "ecchi," which focuses more on nudity, partial nudity, and provocative clothing instead of pure sex.
I really don't have any intention of going any further on the topic of hentai in the Anime section. While I'm willing to admit that this genre of anime does exist, I just don't feel comfortable going much further with it, because minors frequent the Anime section, and the BellaOnline site as a whole strives to be as family friendly as possible.