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It's manga that are generally ero and perverted.

They're usually based off of manga or games or something.

EDIT: The manga Doujin Work kinda explains it all. It's hilarious, too. :3

Well, technically, doujinshi that are derivatives of someone else's work (eg Naruto) are illegal in both the US AND Japan. However, the manga industry usually looks the other way for a variety of different reasons. 1. Doujinshi is where a lot of new talent is discovered. 2. Many artists supplement their salary by producing doujinshi, even though they are serialized mangaka. 3. It is believed that doujinshi can actually stimulate interest in a series, and thus generate more profit for the official works. 4. Doujinshi really don't hurt anyone. They don't take profits away from official companies, because Shounen Jump isn't going to be putting out SasNaru slash anyways...

What Is Doujinshi

Doujinshi is manga created by fans based off of a work that has already been created.

Must of it is comedic, but it is quite varied nonetheless.

why don't you just find in google and its fan made manga and sometimes is by the company or fans but mostly fans just like twinbells doujinshii or

Naruto Yaoi Doujinshi Scanlations & Raws(some you can read here)

Ah, the glory that is Wikipedia:

http://en.wikipedia.org/wiki/Doujinshi

It's usually by fans, and its the porn version of an anime and/org manga. It is of course 18+ rated

it really is fan-made manga. some if it is pure porn.. others are gag and comedic.