An object's design typically begins in a computer aided design (CAD) software system, where its blueprint is created. The blueprint is then sent from the CAD system to the printer in a file format known as a Stereolithography (STL), which is typically used in CAD systems to design 3D objects. The printer then reads the blueprint in cross-sections and begin the process of recreating the object just as it appears in the computer aided design. In the picture below is an example of a 3D printer called the FlashForge.
3D printers are used in many disciplines--aerospace engineering, dentistry, archaeology, biotechnology, and information systems are a few examples of industries that utilize them. As an example, a 3D printer might be used in the field of archaeology to physically reconstruct ancient artifacts that have been damaged over time, thus eliminating the need of a mold.