Naval Units
Unless otherwise noted, all ship counters in the game represent a single vessel. Each ship belongs to a Class and there may be multiple ships in a Class but they are all represented individually.
For instance, suppose you had a destroyer Class called 'Flower'. This Class could consist of four identical ships but all named individually, 'Rose', 'Bluebell, 'Daffodil' and 'Hyacinth'.
Battleship (BB)

Battleships are the primary Capital ships of your fleet. In the time-frame of Imperialism they have reached the limit of their development and will soon be superceeded by the Aircraft Carrier. However, in Imperialism they are still the masters of the seas.
Battleships are your most heavily armed and armoured ships and form the core of your task forces.
Aircraft Carrier (CV)

Now in their ascendency, Aircraft Carriers project air-power hundreds of miles away from their position to attack shipping well beyond the range of conventional guns.
However, the Aircraft Carrier is itself just as vulnerable to such air attacks and considerably less armoured than a Battleship.
Loaded up with fighters (for CAP and escort duties) as well as Torpedo Bombers and Dive Bombers for the attack role, the Aircraft Carrier is slowly changing the face of naval warfare.
Heavy Cruiser (CA)

Building a Battleship represents a significant capital investment. The rise of the aircraft carrier has necessitated the development of escorts with sufficient anti-aircraft firepower to protect the Carriers against air-attack whilst still retaining the heavy hitting power of their predominantly 8" guns. This also makes them good for operating on their own without battleships.
Heavy Cruisers are a good and cost-effective way of projecting sea-power without involving extravagent expenditure of time and resources.
Primarily ships of 10,000 to 15.000-tons and armed with 8" guns,
Light Cruiser (CL)

Although a lot smaller in displacement than a Heavy Cruiser, the Light Cruiser can be produced in large numbers and still pack a punch.
Excellent in the role of anti-aircraft ships the light cruiser can provide close in air-defence while still being used as a combatant due to dual-purpose weapons.
Primarily ships of upto 10,000-tons and armed with 6" guns.
Destroyer (DD)

The Destroyer is a fast, maneuverable, and yet long-endurance warship intended to escort larger vessels in a fleet, convoy or task-force and defend them against smaller, powerful, short-range attackers.
They are fitted with anti-aircraft guns, radar, and forward-launched ASW weapons, in addition to their existing light guns, depth charges, and torpedoes. Destroyers are multi-purpose vessels that are attractive as targets in their own right.
Destroyer Escort (DDE)

The destroyer escort is not nearly as expensive as the fleet destroyer (DD) and much better suited for convoy escort duties. Although they are slower than the DD's (21 knots against 35 knots), they are well armed and most important of all, they can be built much faster. These vessels are the most common submarine hunters in any fleet.
Corvette (DDC)

Designed for anti-submarine duties the Corvette is not ideal for high-seas duties due to it's small size.
Their small size does however make them ideal for coastal duties and close-to-shore escort and anti-submarine roles.
Frigate (FF)

Not quite a Corvette, the Frigate is designed for speed and endurance. The Frigate is designed as an anti-submarine escort.
Getting a convoy through was not just about sinking submarines, it was as much about forcing them down until the convoy got clear. The Frigate can cruise with a convoy, sprint out and lay patterns of depth charges, kills were a bonus.
Minelayer/sweeper (DM)

Keeping the sea lanes free of mines is an important objective. Equally important is the other side of the coin, interdicting the enemy's sea lanes with mines of your own. Both tasks are accomplished by these vessels.
An often overlooked role is that of sweeping ahead of a convoy or task force. Rules for mine-warfare will make provision for the inclusion of minesweepers in such circumstances.
Submarine (SS)

Stealthy, almost totally silent, and deadly to unwary shipping. The submarine is an excellent predator of the seas, hunting down and sinking shipping before disappearing back into the depths.
To counter the threat of submarines, Convoys and Task Forces have 'Escorts', small Destroyers specifically designed for the role of anti-submarine warfare.
Merchant Transport (AK)

Merchant transports are used for the transportation of everything that needs to move by sea except for Oil. This includes the bulk transport of mined Ore, acting as troop transports for Army Groups and Artillery (not involved in amphibious assaults), for the collection and delivery of supplies, munitions, anti-aircraft weapons and steel etc.
Transports collect and deposit their cargos at a Port.
The only exception to this is that they may load/unload cargos from an established beachhead (see Amphibious Assaults and Beachheads).
Merchant Tanker (AOG)

Merchant tankers are used solely for the transportation of Oil from overseas stockpiles to the Homeland for refining into Naval fuel.
Tankers collect and deposit their cargos at a Port.
Amphibious Landing Ship (LSD)

An LSD counter actually represents a small flotilla of such craft that act as a single unit for the transportation of Army Groups and Artillery.
In order to invade independent Islands it is necessary to make an amphibious assult in a coastal area and establish a beach-head. LSDs are the ONLY craft that can land assault troops anywhere other than a Port.