Yeah, I think "doujinshi" is the blanket term of "fan-manga", similar to "fanfics". It's characters from a series that someone's drawn either in an Alternate Universe, or in the same universe as a fan-omake.
I've actually got a few manga titles that have doujinshi's, but they're from the original creator of the manga- but they're so far from being just a plain omake, that they've been titled doujinshi's.
My problem I encounter with these is that I kind of want to read them before buying them, and 99% of them don't seem to function like that.