STL file is a 3D file format that 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 at vizframe.com/3d-models can also be represented by rectangular polygons or some combination of rectangles and triangles. The 3MF also uses triangulated surfaces to represent a 3D model.

What to double-check before 3D printing an STL file:

1) Scale

When you want someone else to print your STL file, it is wise to specify the correct units/dimensions to them clearly. A common error occurs when you create your 3D model in inches, and the 3D printing studio uses mm as a default. 

That means your part could get 25.4x smaller. That’s a big error. Imagine you would like to get a prototype of your screwdriver design (in real-life dimensions), but instead received a prototype so tiny that even a Lego mini-figure would have problems holding it.

2) Resolution

You should always double-check that your STL model is exported with a sufficient resolution of the polygon mesh (that applies to .3mf also). Too low of a resolution will degrade your design.

However, too big resolutions can result in an enormously huge file that will be problematic to handle. When exporting an STL file, you usually have an option to select Binary or ASCII format. Binary format results in much smaller file size so this option is used primarily.

3) Correctly oriented normals

What exactly are normals? Normal is a vector perpendicular to a plane. In a 3D graphic world, there is usually just one normal for each plane (polygon).