"U.S. immigration policy is governed largely by the Immigration and Nationality Act (INA), which was first codified in 1952 and has been amended significantly several times since. At a fundamental level, U.S. immigration policy can be viewed as two sides of a coin. One side emphasizes the faciliation of migration flows into the United States according to principles of admission that are based upon national interest. These broad principles currently include family reunification, labor market contribution, humanitarian assistance, and origin-country diversity. [...] The other side of the immigration policy coin emphasizes the restriction of entry to and removal of persons from the United States who lack authorization to reside in the country, are identified as criminal aliens, or whose presence in the United States is not considered to be in the national interest. Such immigration enforcement is broadly divided between border enforcement--at and between ports of entry--and other enforcement tasks including detention, removal, worksite enforcement, and combatting immigration fraud."
CRS Report for Congress, R45020
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html