These terms are often confused, but each holds a different level of significance.
The National Register of Historic Places is the definitive list of all federally registered historic properties. It includes buildings, structures, sites, objects and districts. Individual structures are listed on the National Register, but entire neighborhoods or areas can also be designated as a National Historic District.
A National Historic District is a district important in American history at the national level of significance and is listed on the National Register. It can also be a historic district owned by the federal government, such as the Gettysburg Battlefield Historic District. To qualify, the area must retain architectural integrity and reflect an aspect of the area’s history. A historical overview of the entire district is needed. The purpose of the overview is to provide a basic background history of the area and to justify the significance of the district. Historic resources survey documentation is required for all proposed districts, which involves photographing and mapping all buildings in the district, recording their architectural characteristics, and assessing whether or not they contribute to the historic character of the district.
The highest level of designation is a National Historic Landmark. Landmarks are properties deemed significant to all Americans because of their exceptional values or qualities, which help illustrate or interpret the heritage of the United States. Designated by the Secretary of the Interior, National Historic Landmarks are a select group of National Register listed properties that rise to national importance due to their exceptional ability to illustrate or interpret American heritage.  If a property is named a National Historic Landmark, it is also listed on the National Register of Historic Places and able to obtain federal historic preservation funding, when available. Only three percent of properties on the National Register are also Landmarks and they are usually owned by private individuals or groups; others are owned by local, state, tribal or federal government agencies. Today, just over 2,500 historic places bear this national distinction. Twelve have been designated in California since 1999.
For more information, visit the National Park Service Website.