Traditional libraries generally collect books and other published items while an archives contains the official records of an organization or the private papers of an individual or family.
Libraries generally maintain a comprehensive catalog listing all of their holdings.
Presidential libraries are not libraries in the traditional sense, but archival repositories for Presidential historical materials. Archival records are the documents created, received, or accumulated by a person or organization in the course of the conduct of affairs, and preserved because of their continuing value. They are the raw primary source material from which history is written. Questions relating to general historical or factual information, biographical information, and compiled, statistical information are usually better answered in a library. NARA does not have an overall index describing all its holdings.