"The mission of the Department of Justice (DOJ) is to 'enforce the law and defend the interests of the United States according to the law; to ensure public safety against threats foreign and domestic; to provide federal leadership in preventing and controlling crime; to seek just punishment for those guilty of unlawful behavior; and to ensure fair and impartial administration of justice for all Americans.' DOJ provides legal advice and opinions, upon request, to the President and executive branch department heads. DOJ prosecutes individuals accused of violating federal laws and it represents the U.S. government in court. The department enforces federal criminal and civil laws, including antitrust, civil rights, environmental, and tax laws. DOJ, through agencies such as the Federal Bureau of Investigation (FBI); the Drug Enforcement Administration (DEA); and the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), investigates organized and violent crime, illegal drugs, and gun and explosives violations. The department, through the U.S. Marshals Service (USMS), protects the federal judiciary, apprehends fugitives, and detains individuals who are not granted pretrial release. DOJ's Bureau of Prisons (BOP) incarcerates individuals convicted of violating federal laws. The department also provides grants and training to state, local, and tribal law enforcement agencies."
CRS Report for Congress, R44424
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html