The .stl or stereolithography format is an ASCII or binary file used in manufacturing. It is a list of the triangular surfaces that describe a computer generated solid model. This is the standard input for most rapid prototyping machines.The description of .stl file format can be easily found elsewhere in the internet. An elaborated description can be found at here. Below is an example of this file format.
facet normal 0.00 0.00 1.00 outer loop vertex 2.00 2.00 0.00 vertex -1.00 1.00 0.00 vertex 0.00 -1.00 0.00 endloop endfacet ... endsolid
Here is an example of using TetGen to read (-p) a polyhedron described in .stl file format (sphere.stl), compute its constrained Delaunay tetrahedralization.
tetgen -p sphere.stlThe picture of the sphere.