The Department of Defense (DoD):  The executive department underwhich the armed services operate.  In addition to active and reserve military personel, the department employs tens of thousands of DoD civilian employees, many of which are retired military or the spouses of military personel.  The largest cabinate level department, it is headed by the Secretary of Defense (SECDEF), he has several undersecretaries, as well as secretaries of the individual armed services (Secretasry of the Army, Secretary of the Navy, Secretary of the Air Force).  In addition to the armed services themselves, the DoD also includes the National Security Agency (NSA), and the Defense Intelligence Agency (DIA).  In Stargate: SG1 the SGC operates within the DoD, but on the authority of the President of the United States.

The United States Army:  The largest branch of the U.S. military, the army traces it's history back to 1776 and the revolutionary war.  Until the mid-20th century it has traditionally been a very small peacetime force which would be rapidly brought up to massive strength once war was declared.  This practice fell into disfavor after World War II when the time needed for a build up of forces was no longer part of modern wars: conventional, or especially nuclear conflict.  From 1940 till the end of the Vietnam War the United States maintained a military draft, but has switched to what most professional soldiers consider a superior and better motivated All Volunteer Force (AVF).  The Army is forbidden by Congress to operate most fixed wing aircraft.

The United States Air Force (USAF):  The youngest branch of the U.S. military, the air force began as part of the army and did not become a seperate service until 1947 following the reorganziation of the military in the National Security and National Defense Acts.  Considered "easy" duty by many other branches of the services, especially the army and marines, the air force has a higher percentage of technical personel and what non-flight combat troops it posseses are either highly specialized (security police to guard and defend facilities and air bases), or special operations (pararescue, combat air control, ect...).  In Stargate: SG1 the SGC is part of the United States Air Force.

The United States Navy (USN):  The Navy is the second largest branch of the armed services and operates some of the most diverse forces and specialities.  Within the navy there are three major combat arms, aviatian (carriers), surface warfare, and submarine.  These arms within the navy can and do act as mini-services which battle for funding and priorities within the naval hierachy.  Most of the time a careful balance is struck in the appointment of senior officers as not to upset any particular faction.  Because of the current state of naval warfare though, the submarine and aviatian branches have taken a senior role in relation to the more traditional surface components.

The United States Marine Corps (USMC):  Part of the Department of the Navy, the Marine Corps traditional duties have involved defending naval vessels from boarding and involvement in amphibious operations.  The smallest of the services, it's members consider themselves an elite force, though the designation is contested by the other services.  A common nickname for marines is "leatherneck" after their early uniform which included a thick leather collar to avoid their thoats from being cut during hand to hand combat.  That aspect of the uniform can also be blamed for the much more derogatory nickname "jarhead" because the uniform is said to have made the marines look like their heads were screwed onto their bodies.  In Stargate: SG1 the SGC employs several Marine combat teams, including SG3.

The United States Coast Guard (USCG):  During peacetime part of the Department of Homeland Security, the Coast Guard reverts to the control of the United States Navy during war.  Coast Guard ships participate mainly in operations close to the United States, but can be found in military operations all over the world.  It is the only armed service with civilian policing power.

The National Intelligence Community:  The blanket term for the verious agencies like the Central Intelligence Agency (CIA), Natioanl Security Agency (NSA), National Reconissence Office (NRO), as well as departments and organizations with an intelligence function like the Federal Bureau of Invesigations (FBI), and the State Department.  Most of these agencies have little clandestine or covert operations components (despite what many believe the NSA has none, it is primarily a signals and communications collection and filtering organ).  The primary controling powers are the Director of Central Intelligence and the National Security Adviser, who are both civilian presidential appointees.   In Stargate: SG1 the NID is presumably an unspecified part of the National Intelligence Community.