STL stands for Stereo Lithography files and are used primarily for 3D printing. They are also used for capturing the outside contours of Free Form shaped objects with 3D Scanning equipment and used for the creation of 3D CAD files.
Scanning creates a massive point cloud in 3D space which the computer connects all the points into 3 sided triangles. This is what an stl. file is. It can then be sliced to create a 3D printing of the part.