The Naval History and Heritage Command website says that the United States Navy uses the letters "USS" to signify "United States Ship." According to the U.S. Naval Sea Cadet Corps (USNSCC) website, this designation was adopted in 1907 and is used to denote Navy ships manned by military personnel. Once decommissioned, the ship is known simply by its name, without the "USS."
The USNSCC says that beginning in 1895, U.S. Navy ships were designated by ship type and number, such as "Destroyer X" or "Torpedo Boat X," with "X" being a hull number authorized by the United States Congress. By WWI, this system had become cumbersome because of the many new types of ships that had been developed.
The current system for designating ships, according to the USNSCC, was instituted on On July 17, 1920, with all existing ships being given a new two-letter code and a hull number in addition to its name and "USS" designation. For example, according to USNSCC, a destroyer tender originally commissioned in 1915 as "USS Melville Destroyer Tender No. 2" in 1915 was re-designated as "USS Melville AD-2."
Naming Navy Ships
The name of each ship is decided by the presiding Secretary of the Navy. Traditionally, battleships are named after states, such as the "USS Ohio," while cruisers are named after cities, such as the "USS Olympia." Submarines are named for sea creatures, such as the "USS Barracuda." During World War I, Franklin D. Roosevelt was the Assistant Secretary of the Navy. His interest in the study of birds prompted him to sign an order giving bird names to the first 36 minesweeper ships in the new "Lapwing" class, including the "USS Owl" and the "USS Robin."