A great number and variety of documents of legal effect and significance are regulary issued by the President, the various offices of the executive branch, and by independent administrative agencies. The President has always had a lawmaking function, usually exercised through proclamations and executive orders, and the executive departments have issued regulations within their administrative jurisdictions. The independent agencies developed in response to a need for reglulation which could not be met by the established units of government.
The predominant forms of administrative lawmaking resemble the primary sources of statutory and judicial law, such as legislative rules and regulations and adjudicative decisions. Administrative law cannot be considered an isolated discipline separate from other forms of lawmaking. Regulations often implement specific statutory provisions, and court decisions are often necessary to clarify the meaning of regulations or to determine the validity of agency decisions.
(from How to Find the Law, 9th ed., Cohen, Berring, Olson, p.261-262)