Doujinshi manga (often shortened to "doujin") are non-professional and/or self published comics. Doujinshi manga can be broken into two broad categories: fan-comics and original works. Fan-comics, much like fan-fiction, are based on established properties and created by fans of those properties. Original works are based on the doujinshi-ka's (creator) original ideas. The only difference between mainstream manga and original doujinshi is that the doujinshi is not published by a publishing company, it is self-published by the doujinshi-ka.

Doujinshi is essentially amateur comic publishing, but not all doujinshi-ka are amateurs. Many professional, published artists continue to self publish doujinshi in addition to their mainstream manga. These doujinshi may be new original works that they chose not to (or are unable to) have published by a larger company. They can also be side-stories that the artists create for fans of their published works.

(definition/source material is quoted directly from Anime News Network's Website)