The Department of Homeland Security (DHS) was created in response to the attacks on the World Trade Center and Pentagon on September 11, 2001. It's primary focus is to protect the United States and territories against terrorist attacks, as well as man-made accidents and natural disasters. Unlike the Department of Defense, which is focused on military actions, the DHS is focused on the civilian realm, protecting civilians within, at and outside the U.S. borders.
The DHS works at the federal, state and local government level, providing grants to state and local governments to help protect against terrorist activity. To date, these grants total $31 billion. The DHS is currently the third largest cabinet of the government and had a budget of $98.8 billion in fiscal year 2011 (though it only spent $66.4 billion). It is lead by the Secretary of Homeland Security, assisted by the Deputy Secretary. Many agencies, advisory groups and various offices make up the DHS, including U.S. Customs and Border Protection, U.S. Immigration and Customs Enforcement, Transportation Security Administration (TSA), U.S. Coast Guard, U.S. Secret Service, Homeland Security Advisory Council, Domestic Nuclear Detection Office, National Cyber Security Center and others.
Also see: Security definitions