Government Agencies carry out much of the day-to-day administration of government. Many of them are part of the various cabinet-level departments, while some operate independently (created either by legislation or executive order).
Most agencies have two core functions: to carry out the results of legislation, and to conduct research on topics that are relevant to their mission. As government agencies, they are compelled to share the results of this research with all American citizens - generally for free.
What is listed here are some of the largest, most well-known agencies. For a more exhaustive list, see the A-Z list of federal agencies at https://www.usa.gov/federal-agencies/a
This wikipedia list is also helpful: Link