A government agency is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight and administration of specific functions, such as an intelligence agency. There is a notable variety of types of agency. Although usage differs, a government agency is normally distinct both from a Department or Ministry, and other types of public body established by government. The functions of an agency are normally executive in character since different types of organisation (such as commissions) are normally used for advisory functions, but this distinction is often blurred in practice.
Source:http://en.wikipedia.org/wiki/Government_agency