While there are many components of the National Guard, including the Air National Guard, the common use of the term is to describe the U.S. Army National Guard. Each state has its own Guard troops, which can be activated by the governor of that state in the case of disaster or other emergency, such as a hurricane or terrorist attack.
The National Guard is supposed to do just what its name says, "guard the nation." Yet, it usually does so one state at a time. The name is somewhat confusing to people who may not remember their civics lessons. Many people assume that because it is called the "National" Guard, it is under federal control.
In fact, the federal government, as well as other state governments, may not take control of troops or send in outside National Guard troops unless the governor of a state under emergency requests outside assistance. In times of trouble, some people express outrage over this fact, especially if the governor does not act quickly, but there is good reason for this policy.
Essentially, this rule is in place, and the Posse Comitatus Act was developed, to ensure that the federal government could not use armed military troops against U.S. citizens. After the Civil War, Congress created the Posse Comitatus Act, intended to protect America's longstanding precedent of keeping military and civilian power separate. The PCA prohibits armed federal forces, including the National Guard, from being used in a domestic manner except under specific circumstances.
There are, however, times when armed forces, including the Guard, may be used for domestic law enforcement. Sometimes referred to as state militias, but not to be confused with the commonly understood definition of a militia, each state's component of this branch of the military can act independently. The governor of each state is basically the "commander" of his or her state's Guard, unless the Guard is called up to federal service.
In a natural disaster, or in response to lawlessness such as rioting, the governor may activate the National Guard to support law enforcement. During an emergency, the governor must also give permission before troops from other states can enter the state, and the governor must give authority to the federal government if requesting federal assistance.
Not to be confused with the Reserves, the National Guard operates in much the same way as the Reserves. The Guard is a crucial source of troops, gear and supplies, as well as experience. Troops are on hand, at the ready when trouble strikes, and the Guard is made up of men and women every bit as dedicated as those in active service.