Army
Army (from French armée) can, in some countries, refer to any armed force. More commonly, however, it is only used specifically to refer to a land force of the military.
Within a national army, an army can also refer to a large formation, usually comprising one or more corps.
Army is also often used in the description or title of military or paramilitary organizations which are not part of a country's official armed forces (and may well be illegal), such as the Irish Republican Army, and also in some non-military organisations organised on a quasi-military basis, such as the Salvation Army and the Church Army.
Field Army
A Field Army is composed of a headquarters, army troops, a variable number of corps, and a variable number of divisions. A battle is influenced at the Field Army level by transferring divisions and reinforcements from one corps to another to increase the pressure on the enemy at a critical point.
National land forces
A national army is usually the arm of the military service which conducts land-based warfare (for example, the United States Army, or the French Armée de Terre).
Most armed forces make considerable distinction between the army or land forces, the navy, and the air force, often maintaining three independent organizations. Many air forces were formerly part of an army; historically, the United States Air Force originated as part of the United States Army, for example. The Chinese People's Liberation Army however combines all of its arms (ground forces, navy, air force, artillery corps, space) under one organization.
Modern armies comprise several branches (also called services, or administrative corps). These may include the combat branches: infantry, armoured, artillery, and combat engineers, as well as the support branches: communications, intelligence, medics, supply, and army aviation (as opposed to a national air force).
Formations
An army can also be a large military organization (formation) comprising one or more corps. A particular army is named or numbered to distinguish it from military land forces in general—for example, the U.S. First 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, "armies" were actually corps-sized formations, subordinate to an army-sized "front" in wartime. In peacetime, a Soviet army was usually subordinate to a military district.
For the hierarchy of land force organizations, see military organization.