Active Duty vs. Reserve or National Guard
What is the difference between someone who is active duty military and someone who is in the Reserve or National Guard?
A person who is active duty is in the military full time. They work for the military full time, may live on a military base, and can be deployed at any time. Persons in the Reserve or National Guard are not full-time active duty military personnel, although they can be deployed at any time should the need arise.
The Reserve
Each branch of the military has a Reserve component and the Reserve are under the command of their respective military branch (e.g., Army Reserve are under the command of the Army). The purpose of the Reserve is to provide and maintain trained units and qualified persons to be available for active duty in the armed forces when needed. This may be in times of war, in a national emergency, or as the need occurs based on threats to national security. Their presence can be called upon to serve either stateside or overseas. The primary job of the Reserve is to fill the gaps in stateside service positions when the active duty forces ship overseas. Members of the Reserve are required to participate in training drills one weekend a month and two weeks per year.
National Guard
The National Guard consists of the Army National Guard and the Air Force's Air National Guard. While federally funded, the National Guard is organized and controlled by state. However, in times of war, the National Guard can become federalized and deployed. The National Guard engages in a number of activities. During local emergencies, National Guard units assist communities endangered by storms, floods, fires, and other disasters. National Guard companies deployed overseas may see combat, but are more often building schools and hospitals, training local peacekeepers, or teaching local farmers more efficient farming techniques and better ways to use of their land. As with the Reserve, the National Guard requires training drills one weekend a month and two weeks per year. National Guard members are given Veteran status if they have served for 30 consecutive days in a war zone.
Deployment
Members of the Reserve and National Guard may be deployed. When scheduled to deploy, they may have extended drill in preparation, resulting in a greater time commitment on behalf of the Service Member. The current conflicts (Operation Enduring Freedom/Operation Iraqi Freedom/Operation New Dawn; OEF/OIF/OND) have an all volunteer force, which could result in multiple deployments (as opposed to previous conflicts like Vietnam when a draft was instituted to create additional military personnel). OIF/OEF/OND have been the largest and longest lasting mobilization of the Reserve and National Guard since the Korean War.
Merchant Marine
The U.S. is also served by the Merchant Marine, a fleet of merchant ships operated by either the government or private sector that are engaged in commerce or transportation of goods during peacetime. It is an auxiliary of the Navy and during wartime can be called to deliver troops and war material. Those who serve in the Merchant Marine are called Mariners (not Marines). They are civilians who are only considered military personnel during times of war; Mariners are given Veteran status if they served in a war.
Sources
http://www.fas.org/sgp/crs/natsec/RL30802.pdf*