The Justice Department (formally, the United States Department of Justice, often shortened to DOJ) is a {cabinet} department of the government. DOJ is charged with enforcing federal law and oversees the entire federal criminal process, from policing (through agencies such as the Bureau of Alcohol, Tobacco, Firearms, and Explosives, the Drug Enforcement Agency, and the Federal Bureau of Investigations) to prosecuting (via the United States Attorney General and Assistant Attorneys General) to housing those convicted (the responsibility of the Federal Bureau of Prisons).

The Justice Department is also charged with defending law in court challenges.

http://www.usdoj.gov/02organizations/