Military reserve force
They have civilian status, except for the days when they are carrying out their military duties. Most reserve warrant officers are former active duty warrant officers. The main sources of reserve warrant officers are military schools and reserve warrant-officers courses.
In countries with conscription, reserve warrant officers are military personnel with the relevant rank who are not on active duty and have not reached the upper age limit. In addition to the upper-age limit, intermediate age limits determine wartime mobilization priority; younger officers are mobilized first. The main sources of reserve warrant officers are promotion during active-duty service or short-term peacetime training, assessment after demobilization, military schools, and reserve warrant-officer courses.

Commissioned officers: In countries with a volunteer military, reserve officers are personnel with an officer's commission who have signed a contract to perform part-time military service. They have civilian status, except when carrying out their military duties. Most reserve officers are former active-duty officers, but some become reserve officers after promotion. The main sources of reserve officers are:

Military schools, colleges and academies, which prepare career officers (who join the reserve after concluding active duty)
Military educational units in civilian higher-education institutions of higher education, such as the US' Reserve Officers' Training Corps
Reserve officer's courses
Direct commission
In countries with conscription, reserve officers are officers who are not on active duty and have not reached the upper age limit. The main sources of reserve officers are:

Training and assessment at the end of conscript service. About eight percent of Finnish conscripts become reserve officers after one year of service.
Military educational units in civilian higher-education institutions, such as military departments (Ukrainian: військова кафедра) in Ukraine and military faculties (Belarusian: ваенны факультэт) in Belarus
Military schools, colleges and academies, which prepare career officers (who join the reserve after concluding active duty)
Reserve-officer courses
In countries with conscription and volunteers, the main sources of reserve officers are:

Military educational units in civilian higher-education institutions such as Russia's military training centers (Russian: военный учебный центр), which prepare officers (who join the reserve after graduation or after concluding active duty)
Military schools, colleges and academies, which prepare career officers (who join the reserve after concluding active duty)
Reserve-officer courses
Training and assessment at the conclusion of conscript service

Advantages: Military reserve personnel quickly increase available manpower substantially with trained personnel. Reservists may contain experienced combat veterans who can increase the quantity and quality of a force. Reservists also tend to have training in professions outside the military, and skills attained in a number of professions are useful in the military. In many countries, reserve forces have capable people who would not otherwise consider a career in the military. 
A large reserve pool can allow a government to avoid the costs, political and financial, of new recruits or conscripts. Reservists are usually more economically effective than regular troops, since they are called up as needed, rather than being always on duty. Preparations to institute a call-up (obvious to adversaries) can display determination, boost morale, and deter aggression. 
Many reservists see voluntary training as merely for supplemental income or a hobby, and so reservists are inexpensive to maintain, their cost being limited to training and occasional deployments. The skills of reservists have been valuable in peacekeeping because they can be employed for the reconstruction of infrastructure, and tend to have better relations with the civilian population than career soldiers.

Disadvantages: Reservists are usually provided with second-line equipment which is no longer used by the regular army, or is an older version of that in current service. Reservists also have little experience with newer weapons systems. Reservists who are retired service personnel are sometimes considered less motivated than regular troops. Reservists who combine a military and civilian career, such as members of the United Kingdom's Army Reserve, experience time demands not experienced by regular troops which affect their availability and length of service.

Forces by country: 

See also: Military reserve
National Guard
State defense force
Militia
Maritime militia

References: 

== Further reading ==