The link in @pascal’s post is where you need to start. The definition of surfaces, polysurfaces and all the other types of geometry starts on the third page.
-Jakob

A curve in Rhino uses NURBS equations to represent a curve in 3D space. Control points are the coefficients used in the NURBS equations. A curve is either open with two ends, or closed.

A polycurve in Rhino is two or more curves joined together. Each pair of curves coincide at one end of each curve. Only two curves can coincide at a point. For many but not all operations a polycurve is treated as a single object. A polycurve is either open with two ends, or closed.

A surface in Rhino uses NURBS equations to represent a surface in 3D space. Control points are the coefficients used in the NURBS equations.

A polysurface in Rhino is two or more surfaces joined together. Each pair of surfaces coincide along an edge of each surface. Only two surfaces may coincide along an edge, but more than two surfaces may coincide at the corners of the surface. For many but not all operations a polycurve is treated as a single object.

A solid in Rhino is a surface or a polysurface which is which is closed. Closed means that there are no edges which do not coincide with another edge. The surface of the solid is represented by the surface or polysurface.

Control points are available for curves, polycurves and surfaces. Control points are not available for polysurfaces.

SubD can be thought of as a type of surface which has different constraints and properties than a NURBS surface

The control points of a NURBS surface are arranged in rows and columns. Each row and each column have the same number of control points. The number of control points in the rows can be different than the number of control points in the columns.

A NURBS surface can be untrimmed or trimmed. A trimmed surface is part of an untrimmed surface. Associated with every trimmed surface is an untrimmed surface.

All untrimmed NURBS surfaces have four edges. One or more edges can have zero length.

The control points of a NURBS surface can be considered as the vertices of a mesh.

The “control points” of a SubD surface can have any arrangement.

A SubD surface cannot be trimmed.

A SubD surface has zero curvature perpendicular to any naked edges.