From the Document: "The term 'reserve component' (RC) refers collectively to the seven individual reserve components of the Armed Forces. Congress exercises authority over the reserve components under its constitutional authority 'to raise and support Armies,' 'to provide and maintain a Navy,' and 'to provide for organizing, arming, and disciplining the Militia....' (Article I, Section 8) There are seven reserve components. [...] The purpose of these seven reserve components, as codified in law, is to 'provide trained units and qualified persons available for active duty in the armed forces, in time of war or national emergency, and at such other times as the national security may require, to fill the needs of the armed forces whenever more units and persons are needed than are in the regular components.' The Army National Guard and the Air National Guard also have an important role under state authority, responding to various emergencies such as disasters and civil disorders."
CRS In Focus, IF10540
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html