The United States government is organized into three branches of Government. These branches are sub-categorized into department, agencies, offices, authorities, commissions, public corporations, and a host of other entities. The U.S. Government Manual identifies all of the parts of the United States government, and provides descriptions, addresses, web information, and other information for all of them. The Manual is issued every year and is available in the reference collection of most libraries. There is a PDF version available on GPO Access at: https://www.govinfo.gov/app/collection/govman/1996_United%20States%20Government%20Manual/1994-95%20Edition
There are two kinds of agencies: departmental and independent. An example of a departmental agency is the National Library of Medicine (NLM), which is a part of the Department of Health and Human Services (HHS). Thus, on an organizational chart of HHS, the NLM will appear as an entity which is under the supervision of the Secretary of Health and Human Services>National Institutes of Health>NLM. The Social Security Administration (SSA), however, is an independent agency (as of March 31, 1995; prior to that time it was part of HHS, and its predecessor department, Health, Education, and Welfare). In another example, The National Labor Relations Board is an independent agency and is not part of the Department of Labor.
The manual is useful, as the government regularly reorganizes itself. The details are found in reorganization plans submitted by the executive to congress and passed (or not) as the case may be. The manual (and older versions of it) gives a portrait of the federal government as it adds, deletes, and consolidates operations within a given year. Appendix B (in the 2003-2004 edition) chronicles the history of Federal Executive Agencies Terminated, Transferred, or Changed in Name Subsequent to March 4, 1933.