The U.S. Department of Homeland Security (DHS) is just that, a "department." There are many "agencies" under DHS such as:
U.S. Customs and Border Protection - inspection, border and ports of entry responsibilities
U.S. Immigration and Customs Enforcement - customs law enforcement, immigration law enforcement, detention and removal, intelligence, and investigations
U.S. Citizenship and Immigration Services - adjudications and benefits programs
The Federal Protective Service (which is part of U.S. Immigration and Customs Enforcement)
The Transportation Security Administration
The Federal Law Enforcement Training Center
Federal Emergency Management Agency
U.S. Coast Guard
U.S. Secret Service
Office of Inspector General
There are more...which one are you looking for specifically ?