Defense Primer: Department of the Army and Army Command Structure [Updated December 11, 2020] [open pdf - 519KB]
From the Overview: "The Department of the Army (DA) is one of the three military departments reporting to the Department of Defense (DOD). The Army's primary mission is to fight and win the nation's ground wars. The Army's mission is both operational and institutional, and it is composed of four distinct components: the regular Active Component (AC), the reserve components of the United States Army Reserve (USAR), the Army National Guard (ARNG), and Department of the Army civilians (DAC)."
CRS In Focus, IF10544
Congressional Research Service: https://crsreports.congress.gov/