Military reserve force
History: Some countries' 18th-century military systems included practices and institutions which functioned as a reserve force, even if they were not designated as such. For example, the half-pay system in the British Army provided the country with trained, experienced officers not on active duty during peacetime but available during wartime. The Militia Act of 1757 gave Britain an institutional structure for a reserve force. Although contemporaries debated the effectiveness of the British militia, its mobilization in several conflicts increased Britain's strategic options by freeing regular forces for overseas theaters.
Reservists first played a significant role in Europe after the Prussian defeat in the Battle of Jena–Auerstedt. On 9 July 1807, in the Treaties of Tilsit, Napoleon forced Prussia to drastically reduce its military strength and cede large amounts of territory. The Prussian army was limited to a maximum of 42,000 men.
The Krumpersystem, introduced to the Prussian Army by military reformer Gerhard von Scharnhorst, gave recruits a brief period of training which could be expanded during wartime. Prussia could draw upon a large number of trained soldiers in subsequent wars, and the system was retained by the Imperial German Army into the First World War. By the time of the German Empire, reservists were given "war arrangements" after completion of their military service with instructions for the conduct of reservists in wartime.

Sources of reserve personnel: In countries such as the United States, reservists are often former military members who reached the end of their enlistment or resigned their commission. Service in the reserve for a number of years after leaving active service is required in the enlistment contracts and commissioning orders of many nations.
Reservists can also be civilians who undertake basic and specialized training in parallel with regular forces while retaining their civilian roles. They can be deployed independently, or their personnel may make up shortages in regular units. Ireland's Army Reserve is an example of such a reserve.
With universal conscription, most of the male population may be reservists. All men in Finland belong to the reserve until 60 years of age, and 65 percent of each age cohort of men are drafted and receive at least six months of military training. Ten percent of conscripts are trained as reserve officers. Reservists and reserve officers are occasionally called up for refresher exercises, but receive no monthly salary or position. South Korean males who finish their national service in the armed forces or in the national police are automatically placed on the reserve roster, and are obligated to take several days of annual military training for seven years.

Uses: In wartime, reserve personnel may provide replacements for combat losses or be used to form new units. Reservists can provide garrison duty, manning air defense, internal security and guarding of important points such as supply depots, prisoner of war camps, communications nodes, air and sea bases and other vital areas, freeing regular troops for service on the front.
In peacetime, reservists can be used for internal-security duties and disaster relief, sparing the regular military forces. In many countries where military roles outside warfare are restricted, reservists are exempted from these restrictions.

Personnel: 

Enlisted personnel: In countries with a volunteer army, reserve enlisted personnel are soldiers, sailors, and airmen who have signed contracts to perform military service on a part-time basis. They have civilian status, except for the days when they are carrying out their military duties (usually two or three days each month and attendance at a two-to-four-week military training camp once per year). Most reserve enlisted personnel are former active duty soldiers, sailors, and airmen, but some join the reserve without an active-duty background. When their contract expires, a reserve soldier, sailor or airman becomes a retired soldier, sailor or airman.
In countries with conscription, reserve enlisted personnel are soldiers, sailors, and airmen who are not on active duty and have not reached the upper age limit established by law. In addition to the upper age limit, intermediate age limits determine the priority of wartime mobilization (younger ages are more subject to mobilization). These limits divide the reserve into categories, such as the Swiss Auszug, Landwehr, and Landsturm. Reserve soldiers, sailors, and airmen are subject to mandatory short-term military training in peacetime, as regulated by law. Reserve soldiers, sailors, and airmen have civilian status, except for military training in peacetime and wartime mobilization. A reserve soldier, sailor or airman becomes a retired soldier, sailor or airman at the upper age limit.
In countries which combine conscription and a volunteer military, reserve soldiers, sailors, and airmen are divided into two categories: reservists and reserve enlisted personnel. Reservists sign a contract to perform military service on a part-time basis. Reserve enlisted personnel are not on active duty, have not signed a contract to perform military service as reservists, and have not reached the upper age limit. Reservists have civilian status, except when they are performing military duties. Reserve enlisted personnel have civilian status, except for military training in peacetime and wartime mobilization. Reservists are first subject to mobilization in wartime. Reserve enlisted soldiers, sailors, and airmen are divided into categories which determine the priority of wartime mobilization (younger personnel are mobilized first), such as Первый разряд (first category), Второй разряд (second category) and Третий разряд (third category) in Russia. A reservist becomes a reserve soldier, sailor or airman when their contract expires, and retires at the upper age limit.

Non-commissioned officers: In countries with a volunteer military, reserve non-commissioned officers are military personnel with relevant rank who have contracted to perform military service on a part-time basis. They have civilian status, except for military duty. Most reserve non-commissioned officers are former active-duty NCOs, but some become reserve NCOs without an active-duty background. When the contract expires, a reserve NCO becomes a retired NCO. The main sources of reserve NCOs are:

Movement from active-duty to reserve service, preserving NCO rank
Military schools, which prepare career NCOs who join the reserve after their active-duty service
Promotion from enlisted rank during reserve service
Reserve NCO courses
In countries with conscription, reserve NCOs are military personnel with 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 the priority of wartime mobilization (younger ages are subject to mobilization first). Reserve NCOs are subject to mandatory short-term military training in peacetime. They have civilian status, except for military training in peacetime and wartime mobilization. A reserve NCO becomes a retired NCO at the upper age limit. Their main sources of NCOs are:

Promotion from enlisted rank during active-duty service, following demobilization
Promotion from enlisted rank during short-term military training in peacetime
Military schools
Reserve NCO courses
In countries which combine conscription and a volunteer military, reserve NCOs are divided into two categories: non-commissioned officers-reservists and reserve non-commissioned officers. Non-commissioned officers-reservists have signed a contract to perform military service on a part-time basis. Reserve non-commissioned officers are not on active duty, have not signed a contract to perform military service as reservists, and have not reached the upper age limit. Non-commissioned officers-reservists have civilian status, except for the days when they are carrying out their military duties. Reserve non-commissioned officers have civilian status, except for military training in peacetime and wartime mobilization. Non-commissioned officers-reservists are subject to mobilization in wartime first. Reserve non-commissioned officers (non-reservists) are divided into categories which determine the priority of wartime mobilization (younger ages are subject to mobilization first) – Первый разряд, Второй разряд, and Третий разряд in Russia. Upon expiration of the contract, a non-commissioned officer-reservist becomes a reserve non-commissioned officer. A reserve NCO becomes a retired NCO at the upper age limit. The main sources of reserve NCOs are:

Promotion from enlisted rank during active duty service, following demobilization
Promotion from enlisted rank during short-term military training in peacetime
Military schools
Promotion from enlisted rank during reserve service
Reserve NCO courses

Warrant officers: In countries with a volunteer military, reserve warrant officers are military personnel with relevant rank who have signed a contract to perform military service on a part-time basis. 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.