The executive branch's definition of homeland security is "a concerted national effort to prevent terrorist attacks within the United States, reduce America's vulnerability to terrorism, and minimize the damage and recover from attacks that do occur." Homeland security activities are divided up into six mission areas: border and transportation security; domestic counterterrorism; protection of critical infrastructure and assets; defense against catastrophic threats; and emergency preparedness and response.
The Department of Homeland Security was created in 2002 and brought together activities previously spread throughout the government including the Coast Guard, the Federal Emergency Management Agency, the Secret Service, the Transportation Security Administration, Immigration and Naturalization Service and the Customs Service. However, some activities outside the Department are considered homeland security; and some activities within the Department are not. The diagram below indicates the breakdown for the proposed fiscal year 2007 budget.
Source: Based on Congressional Budget Office, Federal funding for homeland security, April 30, 2004 [1].
Links:
[1] http://cbo.gov/showdoc.cfm?index=5414&sequence=0