| Careful there, AmSidar. Not all doujin are belong to the hentai genre. Doujinshi is simply a fan made comics that can range from parodies and cross-overs to original works to hentai.
Aside from that, seorang did a wonderful job explaining it. But, a small portion of the manga library was left out, the *-to-manga series (* = numerous sources). Examples of these are Jigoku Shoujo [Hell Girl] (anime-to-manga), Tsukihime and Fate/stay night (eroge [Erotic Game] visual novel-to-manga), and the Suzumiya Haruhi series (light novel-to-manga).
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