The United States Department of the Navy was established by an Act of Congress on April 30, 1798, to provide administrative and technical support, and civilian leadership to the United States Navy. It is headed by the Secretary of the Navy also known as the SECNAV in naval jargon. He is assisted by an Undersecretary of the Navy. The Department was a cabinet position, along with the War Department which contained the US Army until 1947 when the Department of Defense was formed. The navy then became a component of the DOD.
The Department of the Navy consists of Executive offices, mostly located within Washington, DC and is responsible for the recruiting of military and civilian personnel, organizing, supplying, equipping, training, and the mobilizationan and demobilization of the navy and its personnel and assets. The Department also oversees the construction, outfitting and repair of naval ships, aircraft, equipment and facilities.
The Department comprises the two uniformed services, also called the operating forces, the Navy (or fleet) and the United States Marine Corps. There is also the Supporting Services, sometimes called the Shore Establishment. These comprise the dockyards, bases, depots, schools etc.
The highest ranking military officers in the department are the Chief of Naval Operations and the Commandant of the Marine Corps, who are the President's and the SECNAV's principal Naval advisors. They run the military components of the department and sit on the Joint Chiefs of Staff.
US Navy Official website