The National Museum of American History is one part of the Smithsonian Institution. Construction of the NMAH was authorized in the 1950s, and it opened to the public in January of 1964. Some of the exhibits in the museum are permanent, but many others change with the times, such as September 11: Bearing Witness to History, which was on exhibit during my visit in May of 2003.
A brief photo album such as this could never do justice to all that is on display at National Museum of American History. I would encourage you to visit their website, and view many of the other collections that they have online.