After today's lunchtime project presentations the questions came up - What is the difference between a National Historic Landmark and being listed on the National Register of Historic Places? A little sleuthing on the National Park Service website articulated the difference: 
National Historic Landmarks (NHLs) are nationally significant historic places designated by the Secretary of the Interior because they possess exceptional value or quality in illustrating or interpreting the heritage of the United States. Today, just over 2,500 historic places bear this national distinction.  
The National Register of Historic Places (NRHP) is the official list of the Nation's historic places worthy of preservation.  
The more than 80,000 properties listed in the National Register represent 1.4 million individual resources - buildings, sites, districts (i.e. "historic district" with contributing and non-contributing buildings), structures, and objects. To be considered eligible, a property must meet the National Register Criteria for Evaluation. This involves examining the property‚Äôs age, integrity, and significance.