Here is a direct quote from the official US Navy website (an org. chart was included, but it didn't reproduce here):

The following chart presents an overview of the organization of the Department of the Navy. The U.S. Navy was founded on 13 October 1775, and the Department of the Navy was established on 30 April 1798. The Department of the Navy has three principal components: The Navy Department, consisting of executive offices mostly in Washington, D.C.; the operating forces, including the Marine Corps, the reserve components, and, in time of war, the U.S. Coast Guard (in peace, a component of the Department of Transportation); and the shore establishment.