"When federal agencies and programs lack funding after the expiration of full-year or interim appropriations, the agencies and programs experience a funding gap. If funding does not resume in time to continue government operations, then, under the Antideficiency Act, an agency must cease operations, except in certain situations when law authorizes continued activity. The criteria that flow from the Antideficiency Act for determining which activities are affected are complex. [...] This CRS [Congressional Research Service] report is intended to address questions that arise frequently related to the topic of government shutdowns. However, the report does not closely track developments related to the appropriations process for a given fiscal year."
CRS Report for Congress, RL34680
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html