1 - By default, a polygonal model is made of facets, flat or slightly twisted, connected by hard edges.

2 - Phong shading can be used to smooth visually the model. But the initial hardness is still visible on the outlines.

3 - A subdivision surface can be added to the initial model.
A smooth averaged surface made of mini facets is generated
without changing the topology.

4 - With subdivision surface and Phong shading, we get a perfect smooth model, combining ease of use and high visual quality.