Normally they're contained in a box, so the fact that this 3D printer isn't confined means it's theoretically capable of building much larger objects that most. In fact, the owner wants to build a cathedral with it.
It lives in Pisa, Italy, and uses CAD software to create objects designed using the program. Blueprint Magazine describes how it works:
Driven by CAD software installed on a dust-covered computer terminal, the armature moves just millimetres above a pile of sand, expressing a magnesium-based solution from hundreds of nozzles on its lower side. It makes four passes. The layer dries and Enrico Dini recalibrates the armature frame. The system deposits the sand and then inorganic binding ink. The exercise is repeated. The millennia-long process of laying down sedimentary rock is accelerated into a day. A building emerges.
3D printers are still very expensive though, so before you start planning on adding a new extension or granny flat to your house you should definitely weigh up the costs. Having said that, 2010 is apparently going to see the cost lower drastically from the $15,000 or so that they normally cost, with the MakerBot being the cheapest we've seen so far, at $750. [Blueprint Magazine via MAKE]