According to Wikipedia, millwork was "... historically any woodmill-produced ... interior-finish or decorative components." Today's millwork is made of plastics, wood composits as well as solid wood. It is still true that millwork includes interior decorative components. Whether it is an elegant older home, a courtroom, an upscale restaurant or a corporate boardroom, millwork is used to upgrade a plain interior.
In the Revit context, we focus on components easily made with profiles: Baseboards, chair rails, crown molding, casing, picture molding, pencil molding. Yellowbryk's Revit Profile Library is by far the largest available anywhere.