The most commonly used file type for 3D printing is STL (.stl). However, a new file type aims to become the standard for 3D printing. It is called 3MF (.3mf). You can find out more about it at the end of this article. However, I encourage you to read through the basics covering the STL file first. The 3MF file type has some similarities with STL, but the 3MF file is more complex.
STL file is a 3D file format which uses a series of triangles to represent a surface of a 3D model. In the language of 3D graphic designers, the triangles are called polygons. 3D models can also be represented by rectangular polygons or some combination of rectangles and triangles. The 3MF also uses triangulated surface to represent a 3D model.