What are the differences between subsurf and smoth modifiers?
smooth will simply use the normal of each surface on your model to make it look more smooth
but with the subsurf it will interpolate or add extra vertices between existing one and follow a curve which make you model very rounded but the price to pay is the render time
so try to keep the level bewlow 2 or render time becomes very long
EX you start with a 4 vertices and you see almost a circle
Subsurf is a very powerfull tool but it increase the render time quit a lot!