Depends on the software. Usually it is a mathematical description of an object that also includes some form of description of what material/texture/shader makes up the surface of the object. Some software doesn't even use polygons as a way to describe/create an object. (Usually, they revert to a polygon for the rendering process though.) Most of the software can convert between a few differnt kinds of file types to allow interaction between programs. Alot of the programs include animation, rendering, as well as the modeling, but not all. Some programs allow control over machinery that can convert the file description into a physical object, through laser ethcing of a solid, or an ink-jet like building up of layers of resin. A number of manufacturers use this method for rapid prototyping of new items to make. There is also a way to run a pointer like device across an object, or use a camera, and then take the resulting data and turn it into a 3d model, completing the circle.