"Each year, the Department of Homeland Security (DHS) undertakes immigration enforcement actions involving hundreds of thousands of foreign nationals (for definitions of immigration enforcement action terms, see Box 1). These actions include the arrest, detention, return, and removal from the United States of foreign nationals who violate U.S. immigration law. Violations include failing to abide by the terms and conditions of admission or engaging in a variety of crimes such as violent crimes, document and benefit 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 inspection 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 throughout the United States."
Department of Homeland Security: http://www.dhs.gov/index.shtm