A torpedo boat is a relatively small and fast naval ship designed to carry torpedoes into battle. The first designs were steam-powered craft dedicated to ramming enemy ships with explosive spar torpedoes. Later evolutions launched variants of self-propelled Whitehead torpedoes.
These were inshore craft created to counter both the threat of battleships and other slow and heavily armed ships by using speed, agility, and powerful torpedoes, and the overwhelming expense of building a like number of capital ships to counter an enemy's. A swarm of expendable torpedo boats attacking en masse could overwhelm a larger ship's ability to fight them off using its large but cumbersome guns. A fleet of torpedo boats could pose a similar threat to an adversary's capital ships, albeit only in the coastal areas to which their small size and limited fuel load restricted them.
The introduction of fast torpedo boats in the late 19th century was a serious concern to the era's naval strategists, introducing the concept of tactical asymmetric warfare. In response, navies operating large ships introduced firstly batteries of small-calibre quick-firing guns on board large warships for 'anti-torpedo' defence, before developing small but seaworthy ships, mounting light quick-firing guns, to accompany the fleet and counter torpedo boats. These small ships, which came to be called "torpedo boat destroyers" (and later simply "destroyers"), initially were largely defensive, primarily meeting the torpedo boat threat with their own guns outside of the range at which battleships would be vulnerable. In time they became larger and took on more roles, including making their own torpedo attacks on valuable enemy ships as well as defending against submarines and aircraft. Later yet they were armed with guided missiles and eventually became the predominant type of surface warship in modern era.
Today, the old concept of a very small, fast, and cheap surface combatant with powerful offensive weapons is taken up by the "fast attack craft".