3D printing is a method of manufacturing things like models of humans, animals, proteins, commonly used tools, jewelry parts of automobiles, machines and aeroplanes etc. using a computer-controlled printer. Printer means not the ink based printers we use daily. These printers spit materials like metals, plastic, plaster, polymer, resin etc.
Scientifically manufacturing things in 3D printing is called an additive manufacturing technology compared to the normal manufacturing technologies which are called subtractive technologies. 3D objects are generally built out of layers.
