A submarine is vessel and a watercraft that can be used underwater and above water it was especially effective during war for discharging torpedoes. They were made so that countries could attack each other without being seen.
There are 4 main categories of submarines
1) The nuclear submarine is a submarine that is powered by a nuclear reactor. It has an incredible range, which makes it ideal for exploring.
2) Pleasure submarines are generally used by people to view and experience marine life. These submarines are quite small and the rides are generally quite expensive.
3) Submarines that are used to go down to the deepest point in the sea are generally for research purposes, which is why we have special research submarines that scientists use to study the marine life.
4) Military submarines are used for military purposes like war and are used to hold nuclear weapons etc.
Submarines during World War 1 made a very significant impact. Some forces such as the U-boats of Germany were seen in action for the first time. Submarines made it easier for Germany to attack from under water, which is how they sunk three American merchant vessels.
During World War 2 the submarines comprised of less than 2 percent of the US navy but sank more than 30 percent of japans navy including aircraft. (Fact from Internet so it is shown as plagiarized everything else is original.)