STL files are used to represent simple 3D models, defined using triangular 3D faces.
Initially it was introduced as native format for 3D Systems Stereolithography CAD system, but due to its extreme simplicity, it was adopted by a wide range of 3D modelling, CAD, rapid prototyping and 3D printing applications as the simplest 3D model exchange format.
STL is extremely bare-bones format: there are no complex headers, no texture / color support, no units specifications, no distinct vertex arrays. Whole model is specified as a collection of triangular faces.
There are two versions of the format (text and binary), this spec describes binary version.
This page hosts a formal specification of .stl file format of 3D Systems Stereolithography using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.