CGAL users discussion list

Hello,
I have a question about minkowski sum. 
in the example "examples/Minkowski_sum_2/approx_offset.cpp", there is a line 
that computes the sum:

Offset_polygon_with_holes_2 offset;
offset = approximated_offset_2 (P, radius, err_bound);

I want to know what is "offset" exactly? I mean is it still polygon? what is 
the data structure?

what are the functions available for it?

Thanks,