Permanent Legal Immigration to the United States: Policy Overview [December 17, 2012] [open pdf - 409KB]
"The pool of people who are eligible to immigrate to the United States as legal permanent residents (LPRs) each year typically exceeds the worldwide level set by the Immigration and Nationality Act (INA). In an effort to process the demand for LPR visas fairly and in the national interest, LPR admissions are subject to a complex set of numerical limits and preference categories that give priority for admission on the basis of family relationships, needed skills, and geographic diversity. The INA further specifies that each year, countries are held to a numerical limit of 7% of the worldwide level of U.S. immigrant admissions, known as per-country limits or country caps."
CRS Report for Congress, R42866
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html