"Sigh...it can't be helped I guess."
A brief introduction
By definition, hentai is basically explicit (18+) images and videos. These different media types usually originate from Japan, one of the few countries where many people still do hand-drawn comics (known as manga). In Japan it can be used to mean "metamorphosis" or "abnormality." The word "hentai" has a negative stigma in Japan and is commonly used to mean "sexually perverted."
So why is hentai still popular today if the place where it was created looks down upon it so much? The fact of the matter is, the internet has made hentai much more available to people worldwide. All sorts of media can be found on the internet, but hentai specifically has entire sites and databases dedicated to it. Another big reason for the spike in popularity for hentai is that video games have also become a large part of our daily lives. A key feature of hentai is that, because it's fan-made, a very large part of hentai pictures and videos consist of video game characters, or have stories based around them. It serves to fill a fantasy that readers might have, but more so (I think) it satisfies the artist even more. Some titles that are particularly popular in hentai are Street Fighter, Zelda, Mario, Metroid, and most other Nintendo titles.
Hentai is primarily seen in two forms: manga-based comic style and cartoon animation. The manga-based comic style of hentai is known as “Doujinshi” or “Doujin” for short. It is sold in many bookstores and even in some libraries. They do, however, have a strict policy on selling explicit material to minors, and they usually will ask you for identification if you wish to purchase materials from the “blackroom." It reads from right to left, similar to Arabic (and regular manga), and the panel priority is the same as in America, top to bottom. The cartoon animation has almost disappeared off of television in Japan, aside from a few TV channels dedicated to 18+ material. As I mentioned before, talented hentai artists and animators (usually in teams) will find much more business over the internet, commissioning work to people worldwide, rather than have it be looked down upon in Japan.
There is one even more recent form of hentai, and it is in computer CGs and video games. Zone and Hentaikey are some of the oldest online websites that are entirely dedicated to interactive hentai games. If you think about it, the entire idea of hentai is based around fantasy, and these interactive games take it to a whole different level. They were quickly embraced by adult forums worldwide, and many foreign hentai games have been made since.
On a more realistic note, though, hentai is still parodied very much in both American and Japanese culture. This is due to the (usually) high fetish content in most anime, which can make hentai hilarious. Also, you know those really obnoxious screams and ‘moans’ in Japanese porn? Where it sounds like the chick is being mugged? There’s a lot of that, pair that with a cartoon animation and it can be quite humorous.
Hope you enjoyed the rant, cheers!