STL to IGES conversion

I don't know anything about IGES. Most recently I've been using STL.
Accutran and Rhino both can do STL to IGES conversions. I'm trying to
understand what this conversion does and what the result looks like.
Does it maintain the surface informations or is it just a bunch of
splines. I once tried without much success to convert a curvy STL
surface model to IGES. When I read it back into Rhino I couldn't make
sense of the output but may have just done something wrong. This has
become an issue as I understand Bobcad 18 does its 5-axis surface
machining from IGES models.

Any thoughts or experience? Is anyone doing this kind of conversion
already?

Roger