A U-boat is a military submarine used by Germany. German submarines were most active in the First Battle of the Atlantic in World War I and in World War II. Although they could be very effective as weapons against enemy warships, they were most effective in economic warfare (commerce raiding) and enforcing naval blockades against enemy shipping.
The name is an abbreviation of the German Unterseeboot ("undersea boat"). In English, U-boat refers exclusively to the German vessels used during the World Wars. In German, however, the term U-Boot refers to any submarine, including modern and foreign ones.