Dojin's are a group of people with a similar hobby that create something together. Hentai and Fancomics are the most popular. However in Japan its the same as manga.
Easiest way to explain is the visual novel company TYPE-MOON. They were originally a group of 2-3 people that decided "right, let's make a visual novel" and so they came out with Tsukihime, which they sold as a Dojinshi. Dojins literally means self-published works. Both amateurs and professionals alike take part in it, but its mainly amateurs. Fancomics is the most widely popular type of Doujinshi, but a load of circles work on completely original projects. (This is how CLAMP first got started, by creating original Doujinshi and selling them at Comiket)