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USNS  is United States Naval Ship.  These are non commissioned Navy owned ships that are mostly manned by civilian crews.  Types of ships are usualy tankers, supply, oceangraphic and deep sea tugs.  Most carry large hull numbers but can be spotted by the Blue and yellow bands at the top of the funnels.
As a follow on to this, all Commissioned ships in the US Navy are designated as "USS   (name)", which stands for "United States Ship".  These are Naval vessals manned by military crews, and include the warships as well as many auxilary types.
If you're interested in the full list, let me know. I can get it easily out of the "Ships and Aircraft of the US Fleet".  They have changed some over time with a lot of additions.
Tom