From Conflict of Nations Wiki
The lightest class of warship, the corvette is strongest in defending shallow water areas or against submarines not in stealth. Later upgrades allow corvettes to defend themselves against air targets.
A medium size warship, Frigates are particularly strong against air units. Later upgrades allow these units to add stealth detection to their arsenal.
This large warship is particularly strong against surface and deep sea threats, whilst also being proficient against land units and structures. Later upgrades will allow this unit to carry and dispatch missiles.
Considered as one of the most dangerous ships, the cruiser is capable in sea combat as well as carrying and dispatching missiles.
As the name implies, this unit is able to carry various aircraft which are equipped to land on carriers, as well as defend itself against oncoming missiles, fleets and submarines.