This is a post by Katelyn Stevens on PTC site.
In CAD, a surface is an infinitely thin feature used to define complex shapes (think the curvy front end of a sports car). In fact, surface design is generally what gives most consumer products their aesthetic appeal.
How can designers create a manufacturable model when there’s no mass? Typically, you use tools and various editing techniques to thicken or otherwise create the solid model from the surface. Here is what you should know to master working with surfaces: