The US Navy was established October 13, 1775 when the Continental Congress authorized the procurement, fitting out, manning, and dispatch of two armed vessels to cruise in search of munitions ships supplying the British Army in America.

The mission of the US Navy is to maintain, train and equip combat ready Naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas.

Today the US Navy has over 380,000 personnel on active duty, over 150,000 on Ready Reserve and over 184,000 civilian employees.

The Department of the Navy has three principal components:  The Navy Department, consisting or 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; and the shore establishment.

The shore establishment provides support to the operating forces (known as "the fleet") in the form of:  facilities for the repair of machinery and electronics; communications centers; training areas and simulators; ship and aircraft repair; intelligence and meteorological support; storage areas for repair parts, fuel, and munitions; medical and dental facilities; and air bases.

The operating forces commanders and fleet commanders have a dual chain of command.  Administratively, they report to the Chief of Naval Operations and provide, train, and equip naval forces.  Operationally, they provide naval forces and report to the appropriate Unified Combatant Commanders.

(source:  navy.mil)