What are classes of ships and submarines and why are they pigeonholed like that, even accross different countries?
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Classes of ship, like cruiser, destroyer, battleship, carrier, are more descriptions of their function. It’s helpful for a navy to specialize their ships, so that each ship can be very good at one thing or role, while being supported by other ships that can provide other support. It so happens that there are a number of very general, nearly universal roles that most, if not all, navies find optimal.
A destroyer is a small, lightly armed ship that focuses on being fast/agile. The larger or more heavily armed, the slower it would be, so there’s a general trend for destroyers around the world to be built along similar lines.
A cruiser is a ship that is meant to be capable of independent operations far from local bases, a bit of a jack of all trades. It has more firepower and armor/defenses than a destroyer and longer operational range (fuel, supplies necessary to deploy for long periods of time, far from friendly bases). It might not be as fast as a destroyer, but it can punch harder and fight farther away.
A carrier is a ship focused on deploying and resupplying aircraft. Cruisers and destroyers can often do this as well, to a limited degree, but carriers are all-in on aircraft. As such, they are slower (carrying all that jet fuel and bombs/missiles for the planes, as well as the planes themselves) and often less well-armed. But carriers have the size and space to launcher full-size fighter/attack aircraft, so they play a different role than cruisers or destroyers.
Submarines are capable of controllably submerging themselves, so they can be sneakier than other ships. But this means they’re specialized to operate submerged; no aircraft, no guns, only torpedoes and specialized missiles that can be fired from underwater. And since water drags a lot on a ship’s hull and surfaces, submarines generally can’t travel as fast as surface ships can.
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