Army
From the late 17th century, states learned how to finance wars through long term low interest loans from national banking institutions. The first state to master this process was the Dutch Republic. This transformation in the armies of Europe had great social impact. The defense of the state now rested on the commoners, not on the aristocrats. However, aristocrats continued to monopolize the officer corps of almost all early modern armies, including their high command. Moreover, popular revolts almost always failed unless they had the support and patronage of the noble or gentry classes. The new armies, because of their vast expense, were also dependent on taxation and the commercial classes who also began to demand a greater role in society. The great commercial powers of the Dutch and English matched much larger states in military might.
As any man could be quickly trained in the use of a musket, it became far easier to form massive armies. The inaccuracy of the weapons necessitated large groups of massed soldiers. This led to a rapid swelling of the size of armies. For the first time huge masses of the population could enter combat, rather than just the highly skilled professionals.

It has been argued that the drawing of men from across the nation into an organized corps helped breed national unity and patriotism, and during this period the modern notion of the nation state was born. However, this would only become apparent after the French Revolutionary Wars. At this time, the levée en masse and conscription would become the defining paradigm of modern warfare.
Before then, however, most national armies were in fact composed of many nationalities. In Spain armies were recruited from all the Spanish European territories including Spain, Italy, Wallonia (Walloon Guards) and Germany. The French recruited some soldiers from Germany, Switzerland as well as from Piedmont. Britain recruited Hessian and Hanovrian troops until the late 18th century. Irish Catholics made careers for themselves in the armies of many Catholic European states.
Prior to the English Civil War in England, the monarch maintained a personal bodyguard of Yeomen of the Guard and the Honourable Corps of Gentlemen at Arms, or "gentlemen pensioners", and a few locally raised companies to garrison important places such as Berwick on Tweed or Portsmouth (or Calais before it was recaptured by France in 1558).
Troops for foreign expeditions were raised upon an ad hoc basis. Noblemen and professional regular soldiers were commissioned by the monarch to supply troops, raising their quotas by indenture from a variety of sources. On January 26, 1661 Charles II issued the Royal Warrant that created the genesis of what would become the British Army, although the Scottish and English Armies would remain two separate organizations until the unification of England and Scotland in 1707. The small force was represented by only a few regiments.
After the American Revolutionary War the Continental Army was quickly disbanded as part of the Americans' distrust of standing armies, and irregular state militias became the sole ground army of the United States, with the exception of one battery of artillery guarding West Point's arsenal. Then First American Regiment was established in 1784. However, because of continuing conflict with Native Americans, it was soon realized that it was necessary to field a trained standing army. The first of these, the Legion of the United States, was established in 1791.
Until 1733 the common soldiers of Prussian Army consisted largely of peasantry recruited or impressed from Brandenburg–Prussia, leading many to flee to neighboring countries. To halt this trend, Frederick William I divided Prussia into regimental cantons. Every youth was required to serve as a soldier in these recruitment districts for three months each year; this met agrarian needs and added extra troops to bolster the regular ranks.

Russian tsars before Peter I of Russia maintained professional hereditary musketeer corps (streltsy in Russian) that were highly unreliable and undisciplined. In times of war the armed forces were augmented by peasants. Peter I introduced a modern regular army built on German model, but with a new aspect: officers not necessarily from nobility, as talented commoners were given promotions that eventually included a noble title at the attainment of an officer's rank. Conscription of peasants and townspeople was based on quota system, per settlement. Initially it was based on the number of households, later it was based on the population numbers. The term of service in the 18th century was for life. In 1793 it was reduced to 25 years. In 1834 it was reduced to 20 years plus 5 years in reserve and in 1855 to 12 years plus 3 years of reserve.
The first Ottoman standing army were Janissaries. They replaced forces that mostly comprised tribal warriors (ghazis) whose loyalty and morale could not always be trusted. The first Janissary units were formed from prisoners of war and slaves, probably as a result of the sultan taking his traditional one-fifth share of his army's treasure they looted in kind rather than cash.
From the 1380s onwards, their ranks were filled under the devşirme system, where feudal dues were paid by service to the sultan. The "recruits" were mostly Christian youths, reminiscent of mamluks.
China organized the Manchu people into the Eight Banner system in the early 17th century. Defected Ming armies formed the Green Standard Army. These troops enlisted voluntarily and for long terms of service.

Late modern: Conscription allowed the French Republic to form the Grande Armée, what Napoleon Bonaparte called "the nation in arms", which successfully battled European professional armies.
Conscription, particularly when the conscripts are being sent to foreign wars that do not directly affect the security of the nation, has historically been highly politically contentious in democracies.
In developed nations, the increasing emphasis on technological firepower and better-trained fighting forces, make mass conscription unlikely in the foreseeable future.
Russia, as well as many other nations, retains mainly a conscript army. There is also a very rare citizen army as used in Switzerland (see Military of Switzerland).

Field army: A particular army can be named or numbered to distinguish it from military land forces in general. For example, the First United States Army and the Army of Northern Virginia. In the British Army it is normal to spell out the ordinal number of an army (e.g. First Army), whereas lower formations use figures (e.g. 1st Division).
Armies (as well as army groups and theaters) are large formations which vary significantly between armed forces in size, composition, and scope of responsibility.
In the Soviet Red Army and the Soviet Air Force, "Armies" could vary in size, but were subordinate to an Army Group-sized "front" in wartime. In peacetime, a Soviet army was usually subordinate to a military district. Viktor Suvorov's Inside the Soviet Army describes how Cold War era Soviet military districts were actually composed of a front headquarters and a military district headquarters co-located for administration and deception ('maskirovika') reasons.

Formations: In many countries, especially in Europe or North America, armies are often subdivided as follows:

See also: Lists of armies
List of armies by country
List of army units called Guards
List of numbered armies
List of countries by number of military and paramilitary personnel
Military organization
Paramilitary

References: 

External links:  Media related to army at Wikimedia Commons