3D printers create solid objects from various materials. A blueprint for the object is created with CAD or modeling software and sent to the printer via a connected computer. Either the modeling software or a separate piece of software called a "slicer" divides the blueprint into thin layers, and the printer prints these layers one at a time.
Object designs are usually either created by the user or downloaded from the Internet. Blueprints can also be created by a 3D scanner capable of scanning a solid object. An extension commonly used for these blueprint files is .STL, which stands for Standard Tessellation Language.
Materials that can be used include plastic, paper, metals and rubber. The 3D printer usually sprays or squeezes the material onto a platform layer by layer in passes that are similar to the mechanics of an inkjet printer. Layers are usually about 1/10 millimeter in size, but printers exist that are capable of printing even thinner layers. The printer also fuses the new layer with the previous layers as it makes its passes.
The actual printing of an object can take anywhere from a few hours to a few days, depending on a number of factors, such as the type of printer and the object's size.