Two solids, one with extra lines

Here is a picture of two solids. The one on the left was extruded from a rectangle (extrude planar curve), the one on the right from a surface (extrude surface). Why does the one on the right have the extra lines on the faces and what does it mean?

The one on the left is an extrusion object, the one on the right a polysurface. If you explode and rejoin the one on the left it will look like the one on the right. The “extra lines” you see on the polysurfaces are called isocurves. In extrusions they are not shown.

There are some subtle differences between the two, but in theory, they are (should be) more or less interchangeable.

–Mitch

Mitch is spot on.
Here’s a link to the Wiki page with more detail than you probably need:
http://wiki.mcneel.com/rhino/rhinov5status_extrusions