>parody: Original I've never seen that field tagged like that before, can someone who knows a little more about this tell me why this is not just classed under 'manga' then?
EDIT: Oh, so the category refers to method of publication more than whether a work is derivative, gotcha. Thanks Megaton.
Original is for doujinshi works that aren't based off any pre-established IP (Intellectual Property)/s. It's under the Doujinshi category because it's a self published work and not part of a serial/tankoubon.
(I think that's a fairly accurate way of stating things but if someone has more to add please do so.)
EDIT: I am being somewhat broad with my generalizations as people can publish doujinshi on sites like DLsite and DMM but still mostly independent in what their content is.
@PipedPiper: Yes, but its fairly outdated nowadays. Largely replaced by shake your booty and twerking currently.