German U-Boats are submarines. In World War 2, the Germans used
these submarines to attack in groups of three or more. The Germans
called these groups of submarines "wolfpacks". The Americans,
British & Canadians also used the name "wolfpack" to refer to a
group of German U-Boats.
A German submarine was called a U-boat which was short for
Unterseeboot (undersea boat). The Kriegsmarine did not name the
submarines, but simply numbered them (like U-47, for instance).
They are still called Unterseeboot, or U-boot.