they all work by building in layers, hardening some material and then using something else for a support structure (some use the unhardened building material, some use a weaker (or soluble) building material, some use the same material, in a thin lattice formation).

Its been several years, but I used to work at a prototyping company. They have a decent overview of the machines they use, which cover all of the major technologies: http://www.rpc.msoe.edu/machines.php

tocomment, there are a few different technologies that are grouped under the "3D Printing" banner:

"Selective Laser Sintering" is the technology used in the "Art" example. With this a process a thin layer of powdered plastic, metal, or cermaic is laid out on a platform, then a laser then hardens a portion of it. another layer is spread out and the process is repeated thousands of times. In the end you shake of the extra dust and your part emerges. http://en.wikipedia.org/wiki/Selective_laser_sintering

"Stereo Lithography" is another process, from Wikipedia: Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below.

After a pattern has been traced, the SLA's elevator platform descends by a single layer thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006"). Then, a resin-filled blade sweeps across the part cross section, re-coating it with fresh material. On this new liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven. http://en.wikipedia.org/wiki/Stereolithography

"Fused Deposition Modeling" is a third technology that works like a very fine grain softserve ice cream machine. a print head goes over a surface laying down a thin, ribbon like string of materials eventually building up enough layers. http://en.wikipedia.org/wiki/Fused_deposition_modeling

"3D Printing" is more like desktop printing in that you have a substrate and "pixel" of materials are deposited until your design emerges as a finished product. http://en.wikipedia.org/wiki/3D_printing

Hope this helps.