“Raw geometry” consists of edges and faces. An edge is a line, and a face is a thin, thin skin stretched across a set of coplanar edges that form a closed loop. When an edge crosses another edge, they cut each other, and all the pieces stick together. When an edge crosses all the way across a face, it cuts the face. When two faces cross, nothing happens until you select them and apply the Intersect Faces command, whereupon they cut each other along the intersection.
SU raw geometry–edges and faces–is flat and straight. There are no true curves (or arcs or circles). These forms are all simulated by straight-line segments butted end-to-end. There are no curved surfaces either: these are simulated by a network of flat facets approximating the shape of the curved surface.
The edges between facets are “softened,” which is similar to “hidden.” Softened edges do not show up as lines, but they do appear like creases across the surface under certain lighting conditions, so the faceted appearance is still evident. The “creases” are eliminated by “smoothing,” which smoothly blends graduations in light and shade across the entire surface instead of changes in surface shading appearing suddenly (like creases) at the invisible edges. Softening and smoothing are both controlled by the same dialog.