An Army is a term either referring to one of the largest types of formation for a land-based military force, or the entirety of a nation's military force responsible for its land defenses, both of which are composed of soldiers, trained to engage in war.
In modern warfare, a field army is an echelon above corps, but usually more an administrative than operational headquarters. It is unlikely that multiple field armies will ever be in battle, but, in the Second World War, army groups were operational headquarters for multiple field armies.
Not all countries use "army" to indicate the same level of command. Western practice is that a field army is made up of two or more corps, and is commanded by a full general or lieutenant general. WWII Japanese "armies" were of several levels; an unqualified "army" was equivalent to a Western corps, an "area army" to a field army, and a "general army" to an army group.
Armies are most commonly numbered, with the designation written out Third United States Army or First Canadian Army. The Japanese Kwantung Army was actually at army group/theater level. Some countries use honorific, branch, or both in their army names, such as the Soviet First Guards Army or Sixth Guards Tank Army.