Surface models are more realistic looking than wireframe models. They have zero thickness. They can hide background objects so that the perception of depth is obtained.
Surfaces are flat planes.
Curved and rounded surfaces are approximated by lots of small, flat planes. Surfaces can be assigned colors and material properties that may be rendered for added realism. All points must be co-planar, or triangulated surfaces will result. Making a flat face on the sloped portion of the wedge below. The boundary curves can be lines, arcs, circles, plines, or even a point.