Submarine
This class includes both submersibles and true submarines. These are the underwater units which can be used to wreak havoc onto your enemies navy and merchant marine.
Carrier
These are ships which have decks that are able to support aircraft that are capable of landing on them. These types of ships are broken down into two types.
Long-deck Carrier
These are the main fleet carriers and are able to support high performance jets that are lnog deck capable, as well as short deck capable aircraft such as helicopters and jump jets.
Short-deck carrier
These are capable of only taking aircraft that have have a short deck capability suck as helicopters and jump-jets.
Destroyer/Cruiser
These are the gun and missile armed vessels of the fleet and includes destroyer, cruisers and battleships. Some are all round vessels whilst other have a much more specialist role such as surface warfare, anti aircraft warfare and anti submarine warfare.
Frigates/Corvettes
These are smaller ships then their bigger brothen but they do carry out the same roles, although most are geared largely towards the anti submarine escort role. They are also cheaper to build due to their smaller size but do forgoe some combat ability due to this fact.
Patrol/Support
These are vessels such as missile boats and patrol boats which are ideal for small navies/ getting a lot of anti shipping missiles to sea quickly.
Transport
These are vessels that are used to not only transport troops but also to keep the fleet supplied out at sea. This class includes all kinds of supply ships and amphibious warfare vessels, except for LHA's and LHD's which are in the carrier's section as they can embark aircraft.