STL shows the triangled based mesh of a 3D Design. What's the difference between a CAD and an STL file? A CAD is defined by its parameters, whereas an STL file is defined by triangles made of vertices and normals. The CAD file is therefore more precise, whereas the STL is a close approximation. CAD is focused on designing for traditional manufacturing, and does not include information on the outside and inside of the part. An STL does include this important information, which is why it became the standard file format for 3D printing.