Flickr/Chris Zerbes
The majority of people living in the United States descended from European immigrants who arrived as early as the 17th century.  In fact, nearly 48 million people, or about 15.5 percent of the U.S. population, reported German ancestry, according to the 2010 American Community Survey conducted by the U.S. Census Bureau.
The Census Bureau defines ancestry as a "person’s ethnic origin or descent, 'roots,' or heritage, or the place of birth of the person or the person’s parents or ancestors before their arrival in the United States." 
The ACS data represents the number of people who reported each ancestry as either their first or second response.
The ancestries also include groups covered in the questions on Hispanic or Latino origin, such as Mexican.