What is a STL File?

Simply explained, a stl file is a format used by Stereolithography software to generate information needed to produce 3D models on Stereolithography machines. In fact, the extension "stl" is said to be derived from the word "Stereolithography."

A slightly more specific definition of a stl file is a triangular representation of a 3D object. The surface of an object is broken into a logical series of triangles (see illustration at right). Each triangle is uniquely defined by its normal and three points representing its vertices. 
Surfaces are represented by triangles.

The stl file is a complete listing of the xyz coordinates of the vertices and normals for the triangles that describe the 3D object.
 

What makes a "bad" STL file?

In short, a good stl file must conform to two rules. The first rule states that adjacent triangles must have two vertices in common. Second, the orientation of the triangles (what side of the triangle is in and what side is out) as specified by the vertices and normals must agree. If either of these two criteria are not met, problems exist in the stl file. Minor gaps and inconsistencies can usually be fixed by specialized STL-handling software. More significant problems usually require going back to the original CAD model for modifications.

Often a stl file can be termed "bad" because of translation issues. In many CAD systems, the number of triangles that represent the model can be defined by the user. If too many triangles are created, the stl file size can become unmanageable. If too few triangles are created, curved areas are not properly defined and a cylinder begins to look like a hexagon (see example below). 
Faceted stl file translated with coarse tolerance.  File translated with fine tolerance.