Department of Homeland Security was established in response to the tragic and horrific events of 9/11. There was also a movement to restructure the "legacy" INS. Legacy INS referes to the form Immigration and Naturalization Service. Today, the DHS is one of the biggest arms of government. It employs hundreds of thousands of independent contractors as well as its full time staff.
The Different Departments
The DHS has several departments. The first is the United States Citizenship and Immigration Services which oversees 'immigration' benefits and adjudicates those applications. Interestingly, the filing fees that an alien or business pays goes to the running of this organization. It is a government bureacracy primarily responsible for non-immigrant visa petitions, immigrant visa petitions, adjustment of status applications, naturalization applications among many other things that is does. Immigration and Customs Enforcment is primarily responsible for the enforcment of the immigration laws in the Interior of the United States. They remove deportable aliens, enofrce the immigration laws by employer raids, and I-9 enoforcement. The CBP, or Customs and Border Protection are resonsible for keeping our borders safe. The TSA, or the Transport Security Administration is primarily responsble for keeping the transportation infrasturcture safe in the U.S.