The Collaborative International Dictionary of English v.0.48:

Army organization \Army organization\
   The system by which a country raises, classifies, arranges,
   and equips its armed land forces. The usual divisions are:
   (1) A regular or active army, in which soldiers serve
   continuously with the colors and live in barracks or
   cantonments when not in the field; (2) the reserves of this
   army, in which the soldiers, while remaining constantly
   subject to a call to the colors, live at their homes, being
   summoned more or less frequently to report for instruction,
   drill, or maneuvers; and (3) one or more classes of soldiers
   organized largely for territorial defense, living at home and
   having only occasional periods of drill and instraction, who
   are variously called home reserves (as in the table below),
   second, third, etc., line of defense (the regular army and
   its reserves ordinarily constituting the first line of
   defense), territorial forces, or the like. In countries where
   conscription prevails a soldier is supposed to serve a given
   number of years. He is usually enrolled first in the regular
   army, then passes to its reserve, then into the home
   reserves, to serve until he reaches the age limit. It for any
   reason he is not enrolled in the regular army, he may begin
   his service in the army reserves or even the home reserves,
   but then serves the full number of years or up to the age
   limit. In equipment the organization of the army is into the
   three great arms of infantry, cavalry, and artillery,
   together with more or less numerous other branches, such as
   engineers, medical corps, etc., besides the staff
   organizations such as those of the pay and subsistence
   departments.
   [Webster 1913 Suppl.]
   [1913 Webster] armyworm