Site is geographical setting. Structure is the load bearing elements, such as the foundation and skeleton.
Skin is the exterior surface, such as siding and windows.
Services are the circulatory and nervous systems of a building, such as its heating plant, wiring, and plumbing. The Space Plan includes walls, flooring, and ceilings. Stuff includes lamps, chairs, appliances, bulletin boards, and paintings. These layers change at different rates. Site, they say, is eternal.