3D printing is a newer manufacturing process that allows for the creation of a product from a raw material. It uses fiber/filament material that can be made of many substances. The 3D Printer uses the fiber/filament material to build up a physical product in many thin layers of material deposits from a 3D digital model.
Additonal information on 3D Printing
Assembling is the manufacturing process where seperate pieces are put together to create a new product. There are a variety of methods of "attachment". Some of these methods are:
Injection molding is the manufacturing process where plastics pellets are melted and injected under pressure into a closed die, possibly with multiple cavities, to create a product. The closed die used to create such products requires a significant investment of time and money to create.
Additional Information on Injection Molding