Interior finish refers to materials installed to the framing of a home including on walls, ceilings and floors.
An example of interior finish are wall finishes, stairs and railings, posts and columns, flooring, cabinetry and counters, plumbing, interior walls that divide interior spaces, doors, lighting, etc.
An easy way to discern this is that interior finish refers to things that are fixed in place, generally as part of the building process. If you have ever visited a model home, generally everything you would select in the model home showroom to finish the home interior would fall under the category of interior finish.
Interior wall finishes can include textured finishes such plaster and faux finishes, along with wallpapers and affixed wall coverings. However, artwork and window treatments are considered to be decorative rather than part of the interior finishes.