Different materials can be used, more generally ABS plastic, PLA plastic, and metal, but also wax, or even plaster.
The applications of 3D printing are varied, but in all cases aim to concrete visualization of projects, or verification of parts ergonomics.

3D printing works on the principle of a heating nozzle, guided on 3 axes in relation to the part, which mechanically heats and deposits a plastic wire (ABS or PLA are the most used) in successive layers on a tray.
The CAD model is cut into 2D layers before printing.

These software tools export 3D models in the form of files on the standard formats for 3-d printers :
•. STL
•. WRL (VRML)
•. PLY
• 3DS
•. ZPR

The exported file is a mesh made up of a series of triangles oriented in space, which determine a volume in 3D. This mesh must be closed to be regarded as a solid and not only a set of surfaces without thickness. In other words, at this stage, the creation must be ready to spend in the real world, not only on a computer.
Now that the file is in a printable format. You can then enlarge or shrink the file you want to print, guide the element and pilot 3D printer.
The lociel of the printer cut sliced the file of the 3D model in the form of hundreds of digital transverse cuts or layers.
Sample of CAD software producing 3-d files ready to print

The sections of the future object are sent to the machine which begins building immediately.
The machine prints sequentially each layer, one above the other, creating a real object.
Once the machine finishes the last layer, the actual object may be removed.