When most people think of Homeland Security, they think of the people trying to stop terrorists from getting on airplanes. Homeland Security does do that, but they are also involved in a lot of other functions.
For example, Homeland Security is the parent agency of FEMA, the emergency management people who were in charge of the response to Hurricane Katrina.
In addition, Homeland Security is in charge of the ICE -- the immigration people who are supposed to be in charge of enforcing immigration law.
Homeland Security is even responsible for the agents who check agricultural products coming into the country.