"To provide funding for discretionary spending programs of the government, Congress generally uses an annual appropriations process. Under congressional rules, when making decisions about the funding of individual items or programs, however, Congress may be constrained by the terms of previously enacted legislation. The way in which the House and Senate interpret and apply this concept under their respective rules and precedents creates a distinction between authorized and unauthorized appropriations. This report provides a brief explanation of this distinction, and its significance for understanding how appropriations and other legislation work in conjunction to determine how agencies may spend appropriated funds. [...] An authorizing statute that establishes a federal agency often creates statutory duties and obligations for that federal agency (including the responsibility to conduct certain activities such as enforcement of the particular law that the agency is charged with administering). If an authorization of appropriations expires, or if Congress fails to appropriate sufficient funds without explicitly denying their use for a particular purpose, those statutory obligations still exist even though the agency may lack sufficient funds to satisfy them."
CRS Report for Congress, R42098
via Federation of American Scientists: http://www.fas.org/sgp/crs/index.html