The U.S. military has not only active forces but also reserve and National Guard components. The Air Force, for example, features a robust reserve organization and also supports Air National Guard units in all 50 states. Those serving in the Air Guard units are normally under the control and authority of their respective states. However, it's possible to move from an Air National Guard unit to the active duty Air Force.
Air National Guard
Along with the Army National Guard, the Air National Guard is a component of the nation's National Guard force. The Air National Guard is also more commonly referred to as just the Air Guard. Units of the Air Guard are normally under the authority of their states' governors though they're subject to federalization by the president, with control then given to the U.S. Air Force. In most respects, the Air Guard is a mirror image of the active duty Air Force.
Air Guard to Air Force
If you're an Air Guard member, there's a process for becoming an active duty Air Force member. Generally, however, you can't just simply transfer from the Air Guard to the Air Force despite the close relationship both have with each other. Rather, if you're Air Guard and want to become active Air Force, you can seek a release conditioned on you joining the Air Force. Visit an Air Force recruiter first to begin the Air Guard to Air Force process.
DD Form 368
Air Guard members can meet with active duty Air Force recruiters, discuss available openings and then obtain DD Form 368. Entitled "Request for Conditional Release," DD Form 368 is signed by the Air Guard member and her active duty Air Force recruiter. Air Guard units aren't required to approve members' DD 368 conditional release requests, though, and entrance into the Air Force isn't guaranteed, either. Air Guard members who have already been trained normally join the Air Force as prior service candidates.
Prior Service Candidates
Usually, when moving from the Air Guard to the Air Force, you stay in the same career field you were in as a Guard member. However, Air Guard members with job skills not needed in the active Air Force can seek new career training when joining. Once your DD 368 conditional release is approved by your Air Guard unit, you'll continue drilling with it until you actually enter the Air Force. Approved DD Form 368 conditional release requests are good for six months.