"The Department of Defense (DOD) obligates approximately $300 billion annually on defense acquisitions. These funds, drawn primarily from more than 100 distinct DOD appropriations accounts, can be divided into six major categories (or six 'colors of money'): Operation and Maintenance (O&M); Research, Development, Test, and Evaluation (RDT&E); Procurement; Shipbuilding and Conversion; Military Construction (MilCon); and Military Personnel (MilPers). Funds from these accounts generally must be obligated within specific time periods [...]. When an obligation period ends, unobligated funds generally expire. DOD has a further five years to liquidate (spend) or reprogram already obligated funds."
CRS In Focus, IF10365
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html