CAD files have a myriad of formats and corresponding file extensions (example: filename.extension.) With all those formats out there, what is best for you to use? CAD files for 3D printing generally fall into two categories: parametric files (equation driven files that are fully defined - i.e. a circle is actually a circle) and mesh files (made of points and triangles - i.e. a circle is thousands of tiny triangles.)  Here's a great guideline. to help you get started.