When talking about doujinshi, you've gotta think of it in degrees.
There's romantic doujinshi, which is just that. A love story created by a fan.
There's ero-doujinshi, which is a more passionate love story. Lot's more kissing and bodily contact, but not explicit in nature.
There's ecchi-doujinshi, which is yet another step up. Nudity is guaranteed, maybe with a bit of sexuality here and there, but nothing incredibly graphic. Think 'sensual touching'. Of course, this is like a building block, so the aspects of the first two are in play. The building block tops off here, as the last one is the most diverse and sporatic.
Then there's hentai-doujinshi. This is probably the one you'll think of first. This one may or may not have any romantic, or even fan-fic ties into it, as it may just be someone who thinks Tsunade and Sakura are hot, yet they don't know a lick about the series itself, and they've just portrayed them in graphic *** acts. This can be as "light" as "normal" ***, or it can become as weird as, say, "vore" (the act of swallowing a large part of something in "some" sense), and it can get as graphic as "guro" (extreme violence). If you think 'anime porn' is bad, then stay far away from this genre.
That should clear some things up. Doujinshi is just a certain type of fanfiction. I also feel I should point out that, in the case of Japanese doujinshi, that the age of consent in Japan is 16, so don't be shocked should you come across ecchi or hentai doujinshi displaying people who you know to be this age with a little more skin showing.