Warships (Civ5)/Civilopedia

While many ships are engineered to cross oceans quickly or take the greatest amount of cargo in the smallest possible space, warships were designed for one purpose alone - destroying other ships. Besides obviously being armed with some form of weapon (traditionally things like cannons or rams), warships can also withstand more damage and are much more maneuverable than something like a merchant or passenger vessel.
As naval technologies have advanced, so has the warship. Galleys and triremes were early forms of warships capable of ramming enemy vessels, but usually combat was performed by the army on board. However, once cannon became capable of quick reloads, large sail-driven ships were employed which held rows of cannons along each side of the craft. Eventually these wooden ships were given up for ones made of iron and steel, and sails were abandoned for steam and nuclear power. While modern warships bear little resemblance to the earliest galleys, they still keep to the same basic design principles - be capable of destroying the other guy first, before he can take out you.