I wouldn't know what a horn enclosure is either. What I do know is that some enclosures have a cone shaped port instead of a straight one. If you do it right, a cone shaped port can be equal to several straight ports, again giving you more options in enclosure building. But I'm not sure about this one, so don't shoot me for it if I'm wrong.

A passive enclosure I think, is an enclosure that makes use of a passive radiator, as I said before. A passive radiator is, as the name may imply, a driver without a motor (no magnet, pole piece, backplate and cylinder)... It makes use of the pressure and airflow coming forth from the actual "active" driver.