There are four main categories of historic resources used by the National Park Service: building, site, structure, and object. The following are the definitions used by the National Parks Service:
- Building: A resource created principally to shelter any form of human activity, such as house.
- Site: Location of a significant event, a prehistoric or historic occupation or activity, or a building or structure, whether standing, ruined, or vanished, where the location itself possesses historic, cultural, or archaeological value regardless of the value of any existing structure.
- Structure: A functional construction made for purposes other than creating shelter, such as a bridge.
- Object: A construction primarily artistic in nature or relatively small in scale and simply constructed, such as a statue or milepost.