The word submarine was originally an adjective meaning "under the sea". Some firms who make diving gear but not parts for submarines, called their work "submarine engineering". "Submarine" as a noun meaning a submersible craft originated as short for "submarine boat" and older books such as Twenty Thousand Leagues Under the Sea always use this term. Also, some people simply say 'sub' instead of saying the entire word 'submarine'.

Submarines encompass one of the largest ranges in capabilities of any vessel. They range from small one- or two-man vessels that can examine the sea floor for a few hours to the Russian Typhoon class, which can remain submerged for 6 months and carry nuclear missiles capable of destroying multiple cities. There are also specialized submarines such as rescue submarines (like the DSRV or Priz) and tiny one-person human powered subs intended for competitions between universities. An older device for use in underwater exploration, salvage, construction and rescue is the diving bell. A specialized form of submarine capable of extremely deep dives is the bathyscaphe.

Submarines are typically referred to as "boats" even though most modern submarines should technically be called "ships". The term U-Boat is sometimes used for German submarines in English. This comes from the German word for submarine, `U-Boot`, itself an abbreviation for Unterseeboot ('undersea boat').

The vertical structure, usually located amidships, houses communications and sensing devices as well as periscopes. In modern submarines it is the "sail" in American usage ("fin" in the Royal Navy). A submarine's "conning tower" was a feature of earlier designs: a separate pressure hull above the main body of the boat that allowed the use of shorter periscopes.