Homeland security refers to the collective efforts and strategies implemented to safeguard a nation's territory, citizens, and infrastructure from threats, including terrorism, natural disasters, and cyberattacks.

Homeland security focuses on protecting a nation's territory, citizens, and infrastructure from various threats within its borders, including terrorism, natural disasters, and cyberattacks. It involves proactive measures such as risk assessment, preparedness, and response coordination.

However, homeland defense is concerned with safeguarding a nation's sovereignty and territory from external threats. It involves military and defense operations aimed at preventing and countering attacks originating from outside the country's borders, including defense against conventional military aggression and asymmetric threats.

To sum it up, homeland security deals with threats within a nation's borders, while homeland defense addresses threats originating from outside those borders.

Some of the major agencies within the U.S. Department of Homeland Security include:

Transportation Security Administration (TSA): Responsible for securing transportation systems, including airports, seaports, and mass transit.
U.S. Customs and Border Protection (CBP): Oversees U.S. borders and ports of entry, including border security, customs, and immigration enforcement.
Federal Emergency Management Agency (FEMA): Coordinates response and recovery efforts to natural disasters, including providing disaster assistance and supporting state and local emergency management agencies.
Federal Law Enforcement Training Centers (FLETC): Provides training to the federal law enforcement community, helping officers fulfill their criminal justice responsibilities to ensure the safety of people, property, and institutions.  
Immigration and Customs Enforcement (ICE): Enforces border control and customs laws, including investigating and detaining individuals who violate these laws.
United States Secret Service: Primarily responsible for protecting the president, vice president, and other high-ranking officials, as well as investigating financial crimes and protecting the nation's financial infrastructure.
Cybersecurity and Infrastructure Security Agency (CISA): Works to enhance cybersecurity and protect critical infrastructure from cyber threats.
U.S. Citizenship and Immigration Services (USCIS): Ensures lawful immigration to the United States and oversees the process for individuals who wish to seek U.S. citizenship.
U.S. Coast Guard: Protects maritime interests and enforces maritime laws, including search and rescue operations, drug interdiction, and maritime security.

A number of other offices and support services operate under DHS as well, including the Management Directorate, Science and Technology Directorate, Countering Weapons of Mass Destruction, Office of Intelligence and Analysis, Office of Homeland Security Situational Awareness, the Office of Health Security, and ombudsman offices.