Original: Among the many DOD agencies are the Missile Defense Agency, the Defense Advanced Research Projects Agency (DARPA), the Pentagon Force Protection Agency (PFPA), the Defense Intelligence Agency (DIA), the National Geospatial-Intelligence Agency (NGA), and the National Security Agency (NSA).
Simplified: The Department of Defense is made up of the Department of the Army, the Department of the Navy, the Department of the Air Force, the National Security Agency, and the Defense Intelligence Agency.