What is a class of ships or submarines?

A class is a group of ships built to essentially the same design. There may be some small differences, often depending on the shipyard where the vessel was built.

Classes are normally named after the first ship in the design that was ordered. In an oddity, the USS Drum (SS-228) was the first Gato submarine completed and commissioned. However, the USS Gato (SS-212) was the first of the design ordered and the class is named after her.