"This Office of Immigration Statistics Annual Report presents information on the apprehension, detention, return, and removal of foreign nationals during 2008. […] Each year, the Department of Homeland Security (DHS) undertakes immigration enforcement actions involving hundreds of thousands of foreign nationals. These actions include the arrest, detention, return, and removal from the United States of foreign nationals who are in violation of the Immigration and Nationality Act (INA). Violations include losing legal status by failing to abide by the terms and conditions of entry or by engaging in crimes such as violent crimes, document fraud, terrorist activity, and drug smuggling. Primary responsibility for the enforcement of immigration law within DHS rests with U.S. Customs and Border Protection (CBP) and U.S. Immigration and Customs Enforcement (ICE). CBP is responsible for the inspections of all arriving persons and conveyances at ports of entry and the deterrence or apprehension of illegal immigrants between ports of entry. ICE is responsible for enforcing immigration laws within the interior of the United States."
United States Department of Homeland Security: http://www.dhs.gov/