Originally Posted by mr henderson
Not really telling us a lot there, though, are you? That sounds more like a difference in how it works rather than what the difference means to the user.
In a 3D CAD, a sphere is a sphere, an exact mathematical representation. In a 3D modeler, a sphere is an approximation made out of many little triangles, like here: http://en.wikipedia.org/wiki/Polygon_mesh
All dimensions involving curved lines or surfaces will be approximate when working with a mesh, and exact when working with a solid model.
That said, Sketchup is great for many CAD-like applications. My brother designed a gazebo in Sketchup prior to building it out of wood and polycarbonate. Worked a treat.