In the United States, there are Army and Air Force reserve components respectively called the [Army] National Guard and Air National Guard; "National Guard" without qualifiers is usually assumed to mean the Army Guard. Guard forces are under a dual command: state governors when not "Federalized", or part of the U.S. military when they are. There are no Naval or Marine equivalents.
Non-federalized Guard units carry out domestic emergency operations under the control of governors, such as responding to storms and forest fires. They may also respond to riots and civil disturbances, although the law becomes more complex here. In some cases, non-federalized units may respond outside the state under governor-to-governor arrangements with state-to-state reimbursement; the California and Oregon Guards, for example, responded to Hurricane Katrina in Louisiana. Each state has a major general commanding the state Guard units, called the adjutant general.
From the national perspective, they are part of the Reserve Components, which also includes the Army, Navy, Air Force, and Marine Corps Reserves, which are always under Federal control. The National Guard Bureau, which funds and oversees both National Guard programs, is administratively part of the Department of the Army.
In principle, Guard units have the same equipment and training as the active duty components. The Air National Guard tends to come closest, as many of the members are in the civilian aviation industry and have daily jobs that keep flying or maintenance skills high. The transition from an airliner to a military transport can be slight, but ANG fighter and bomber pilots have a good reputation.
Army National Guard units take longer to come to combat readiness than full-time units. There has been some pressure to move then, under the restructuring of the United States Army program, out of combat arms, but, as one state officer said, "we like our tanks." Army Reserve under the Total Force Concept, were deliberately assigned, almost exclusively, to combat support and combat service support.
Especially with the duration of the Iraq War and the reductions in Federal forces, Guard forces have been in combat roles longer than in any conflict since the Second World War.