Technically 3D Printers use a Plastic Resin.
Here's the way some of these work:
1. You have a vat of liquid resin. This resin 'hardens' when exposed to a certain wavelength of light.
2. The printers blasts lasers at a point in 3D space. At that point, the proper light wavelength is formed a piece of the resin is hardened.
3. This process continues from the bottom up, essentially 'growing' a solid resin inside the liquid, from the bottom up. Sometimes you have to design supports, which you will then break off.
An example (and video) of such a machine can be found here:
**clicky**This is the opposite of the conventional method of manufacturing models, which is milling, where:
1. You have a solid stock of a material (let's pick wood, this time)
2. you have a machine with an endmill (like a drill-bit tip, only it's round so it can cut across as well as in/out), which starts milling the material out from the stock. The arm of the machine moves in specific movements, called a 'tool path', which pass over the wood, removing a little each time.
3. Eventually, enough material is removed from the stock to have a 'roughing' pass of the model. Either a tool change happens to a smaller endmill, or a second 'finishing' pass happens, which makes the model resemble the CAD mechanical.
To answer Lelygax's (and Pfil's) questions: The way you make a piece with joints is, you design a 3D CAD model of separate pieces, and have someone assemble them once they're done. Another way is to design the piece so that it has pieces you can break off, and use them as supports for the thing you're growing... but that's really unorthodox.
Let it be known that, at least in manufacturing (and definitely jewelry manufacturing), a 3D Printer is good for making the prototyping and production models, which are then sprued, and they themselves are used to make a rubber mold (the sprue is where you shoot the wax to make wax models). The rubber mold is used to mass-produce thousands of wax models, which are then wax-glued to a wax vein and used in Investment Casting (imagine a wedding ring made of wax, now imagine if each piece were a leaf in a wax tree... someone uses wax to glue these leaves to the wax branches of a tree, then the tree is dipped upside down in an investment... usually a material similar to cement). You put the investment in an oven upside down, which hardens the investment and melts/removes the wax. Once the wax is completely removed, you remove the mold from the heat, and using the same hole where the 'tree root' of the wax was, you begin to fill the chambers leftover from the wax with your true material - in this case, melted gold, or silver - and then leave it to set.
Once it's set and the gold/silver is solid, you 'break the mold' leaving a pretty gold 'tree' with rings for branches. You cut the rings from the branches, re-melt the branches and stump, and tumble/polish/work the rings for production. Using this method, you can use one model to make thousands of rings.
Here's a webpage explaining how Investment Casting works:
**Clicky**I would imagine the best way to build a 'house' would be to design interlocking models and join them together after the fact. A 3D printer is not a very large device. I would imagine, at most, you'd be able to build something the size of a computer monitor or a large block or something. They don't make house-sized 3D printers (that I know of).