Well, there's good news and bad news. The good news is that you're right when you think that hentai is a form of manga, which is a particularly Japanese style of comics and comic book art, typified with large eyes and various other aspects. Anime, of course, is the movie version of manga, again with those same exaggerated eyes and physical dimensions.

Just as Western comic book and cartoon art got pulled into more adult themes with the release of Heavy Metal and Fritz the Cat (among many, many others), so does hentai represent the adult and mature themes in the world of manga.

Specifically, hentai literally means:

"A Japanese slang word meaning perverted or perversion. In the world of manga and anime, hentai refers to the adult oriented titles that present extreme, graphic sexual imagery. A common Western stereotype concerning manga and anime is that these artforms are filled with graphic sex. While such imagery exists (and has always existed in Japan), hentai titles make up just a small niche market in Japan's massive manga and anime output."

Before we go too far into that (and don't worry, we won't talk about the more peculiar stuff - this is a family site, after all) let's dig up a useful definition of manga too:

"In 1814 the famous Japanese artist Hokusai created a book of black & white sketches that he called manga (involuntary sketches). In recent Japanese history the word has come to describe those small illustrated books of black & white ink drawings that tell a series of stories. Graphic novel would be the closest translation for "manga" though they are not at all like novels in the Western sense, nor are they comparable to Western comic books."

If you'd like to learn more about hentai, you can certainly Google hentai or, if that's too much, "hentai store" will give you some useful results. Make sure you look both at the so-called organic or natural search results and those that are part of the paid advertising area. Both have useful sites if you want to learn more.