Hi Pat, Colin - both STL and OBJ contain a similar type of polygon mesh object, but OBJ has more information in it that is more suitable for rendering, stuff like texture coordinates, more than 3-sided polygons, shading information, ... STL contains only plain triangle facets and no other types of information.

Basically though, there are different ways that 3D objects can be represented on the computer.

MoI uses a system called NURBS which works with mathematical curves.

A different system is a polygon mesh type object, which is a system that defines an object by using a larger number of small flat polygon facets.

Each different system can have their own particular advantages and disadvantages.

Polygons are used frequently in the entertainment industry - they are kind of easy to squish and deform for making creatures and face type shapes.

NURBS are used frequently in the CAD and manufacturing industry since having true curved shapes allows for a more exact definition of things like spheres, circles, cylinders, ....

It is possible to convert NURBS objects into polygons by dicing up the smooth surface into smaller facets - this is what MoI does when you save to an .obj or .stl file. It is not very easy to go the other way from a lot of small facets into smooth NURBS surface again though.

- Michael