From the Census Bureau:
The Population Projections Program produces projections of the United States resident population by age, sex, race, and Hispanic origin. The 2012 National Projections are based on the July 1, 2011 population estimates, which are based on the 2010 Census, and provide projections of the population for July 1, 2012 to July 1, 2060. The projections were produced using a cohort-component method and are based on assumptions about future births, deaths, and net international migration. The Census Bureau releases new national projections periodically.
The 2012 National Projections include a main series and three alternative series. These four projections series provide results for differing assumptions of net international migration. The main series, referred to as the Middle series, was released in December 2012. The alternative series, released in May 2013, were based on assumptions of low, high, and constant levels of net international migration. All other methodology and assumptions, including fertility and mortality, are the same as those used in the Middle series. The three alternative series are useful for analyzing potential outcomes of different levels of net international migration.