The Army’s mission is to fight and win our Nation’s wars by providing prompt, sustained land dominance across the full range of military operations and spectrum of conflict in support of combatant commanders. The Army is composed of two distinct and equally important components: the active component and the reserve components. The reserve components are the United States Army Reserve and the Army National Guard.
Regardless of component, The Army conducts both operational and institutional missions. The operational Army consists of numbered armies, corps, divisions, brigades, and battalions that conduct full spectrum operations around the world. (Operational Unit Diagram and descriptions) The institutional Army supports the operational Army. Institutional organizations provide the infrastructure necessary to raise, train, equip, deploy, and ensure the readiness of all Army forces. The training base provides military skills and professional education to every Soldier—as well as members of sister services and allied forces. It also allows The Army to expand rapidly in time of war. The industrial base provides world-class equipment and logistics for The Army. Army installations provide the power-projection platforms required to deploy land forces promptly to support combatant commanders. Once those forces are deployed, the institutional Army provides the logistics needed to support them.
Excerpted from the Army’s official web site.