Subterranean termites are the most common type of termite found in the midwest, and are also the most destructive.
They usually enter the home from below, via wood in contact with soil, or tubes they build to keep them from being exposed from soil to wood members of the home. Once they've found the framing of a home, they can cause extreme damage.
I recently inspected a (newer and otherwise very well kept) home in which they had eaten away top and bottom plates in about half of the homes exterior walls, some interior walls, and many ceiling joists. They had entered the home from a back wall with landscaping mulch above the bottom plate, and from below the garage floor. Finding the full extent of the damage would require removing drywall in all these areas (roughly half of home), to inspect the condition of framing materials behind it. Repairs would include replacement or bolstering of damaged materials, and of course termite treatment.
Often, little to no evidence of their presence is visible to the untrained eye, but (what look like) vertical dirt tubes on a low concrete wall, or hanging tubes from joists in a basement, are telltale signs of infestation. They may also dislike certain wood in a home, and build tubes past it to other timber they prefer.
They leave behind hollowed out framing, with the structural strength below corrugated cardboard. The outside of the materials can look fine, while damage to the interior of the board is significant.