U-boat is the anglicization of the German word U-Boot, itself an abbreviation of Unterseeboot (its literal meaning: "undersea boat"), the German word for submarine. The primary targets of the U-boat campaigns in both world wars were the merchant convoys bringing supplies from Canada and the United States to Europe. Austrian submarines of World War I were also known as "U-boats".

The distinction between U-boat and submarine is common in English-language usage but unknown in German where the term U-Boot refers to any submarine.

A submarine is a specialized watercraft that can operate underwater. Military submarines, first widely used in World War I, are used by all major navies today, especially the American and Russian navies. Civilian submarines and submersibles are used for marine and freshwater science and for work at depths too great for human divers.

Nuclear powered submarines and other large submarines are classed as ships, but are customarily referred to by their crews as "boats".

Submarines encompass one of the largest ranges in capabilities of any vessel. They range from small two-man vessels that can examine the sea floor for a few hours to the Russian Typhoon class, which can remain submerged for half a year and carry enough nuclear missiles to destroy hundreds of cities.