Short answers, pulled from the story.
A National Historic Landmark is a building, district, object, site, or structure officially recognized by the United States government for outstanding historical significance. The National Register of Historic Places contains over 90,000 listed places, while only about 2,500, roughly three percent, have earned the higher NHL designation.
More than 2,500 National Historic Landmarks have been designated. They are found in all 50 states, Washington, D.C., U.S. territories, U.S.-associated states, and even one site in Morocco.
The origins of the program trace to the Sergeant Floyd Monument in Sioux City, Iowa, which marks the site where Sergeant Charles Floyd died during the Lewis and Clark Expedition in 1804. It was officially designated on the 30th of June, 1960.
The U.S. secretary of the interior designates NHLs based on six criteria: sites of nationally significant historical events, places where prominent persons lived or worked, icons of ideals that shaped the nation, outstanding examples of design or construction, places characterizing a way of life, and archaeological sites able to yield significant information.
New York State has the most NHLs of all 50 states. Pennsylvania and Massachusetts round out the top three, and together these three states account for nearly 25 percent of all NHLs in the country.
The White House, the United States Capitol, and the United States Supreme Court Building are all designated as National Historic Landmarks but are specifically legislated exceptions to the rule that NHL designation triggers automatic inclusion on the National Register of Historic Places.