3D printing is also known as additive manufacturing. The well known example of additive manufacturing would be injection molding. Plastic is melted down, and the liquid plastic is pushed into a mold, and is allowed to cool into the shape of the mold. 3D printing works somewhat similar to 2D printer.
One of the type of 3D printing is fused deposition modeling FDM printing. It is type of freeform injection molding. In this type of printer, printing head carries an extruder, which melts and forces out plastic through a nozzle. This plastic is used to trace an outline. Then it is allowed to cool. Another layer of plastic is extruded on top of this layer. By allowing the plastic to cool layer by layer, the printer gets rid of the need to have a mold to make the shape. The main benefit is that you don’t need to spend extra time carving out a mold to begin with.