From the Document: "The Constitution gives Congress the power of the purse in Article I, Section 9, which provides that 'No money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law.' In order to fulfill this duty Congress annually considers appropriations measures, which provide funding for numerous activities--such as national defense, education, and homeland security--consistent with policies and priorities established through legislation such as the National Defense Authorization Act. The congressional appropriations process includes various rules and practices that Congress has adopted to distinguish appropriations measures and facilitate their consideration. These measures generally provide funding authority that remains available for a single fiscal year (October 1 - September 30)."
CRS In Focus, IF10514
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html