Solid modeling is a subset of
3d modeling where (usually
binary) operations on solids are used
to construct the final, solid object.
This is different from other types of 3d modeling in that
the object is represented as a solid object, rather than
just the surface of the object.
Operations used to construct the solid include
extrusion, where a face is moved through a vector
to create a prism, or revolution, where a cross section
is revolved about an axis. Binary operations include
joining two solids into a single solid, or calculating
the intersection to find the volumes common to both,
or a cut which puts a hole in one object by removing the volume of the other.
Animation programs and many art programs use surfaces or wire frames
rather than solids, as the concern is the superficial look
of the object.
Solid modeling is used primarily in areas where the
volume of the object is of concern, rather than the surface. The first major solid modeling package was BRL CAD which would trace solid objects with rays (any ray--light, x-ray, sound, missles, whatever).
Current commercial solid modeling packages include SolidWorks, Pro/Engineer, SDRC I-DEAS and other Engineering CAD and CAD/CAM packages.