Planar Nef Polyhedra are whatever one can construct with a finite number of Boolean operations on halfspaces. See the user manual pages for a detailed introduction.
CGAL::Extended_cartesian<FT>
CGAL::Extended_homogeneous<RT>
CGAL::Filtered_extended_homogeneous<RT>
CGAL::Nef_polyhedron_2<T>
CGAL::Explorer
CGAL::Topological_explorer