STL describes the surface of a three-dimensional object as a mesh of triangles. It carries no colour, no texture and no units, which is both why it is so simple to write and why so much information is lost on the way out of a CAD package.
It originated with 3D Systems' stereolithography machines in the late 1980s and became the common currency between modelling software and 3D printers, so nearly every slicer accepts it. Two layouts share the extension: a human-readable ASCII form and a much smaller binary form.
Watch out for the extension clash: application/vnd.ms-pki.stl is a Microsoft Certificate Trust List and has nothing to do with 3D models, despite also using .stl.
File types
.stl
Also appears as
Not known to appear as any other types