Classes of Ships

The way warships are classified tends to be controversial, and there is no set definition for the meaning of a term. Everyone have their own views with their ships. For example Russian destroyers on which I served for many years are known as SNF (Soviet Naval Frigate) in USSR. Since modernisation of navies size of the ship doesn’t matter in classifying the ships. However size was the main factor in the early days in classification. Secondly weapon carried and purpose of ship.

The World’s Navies classifies the ships as follows:

Cruisers: +10,000 tons
Light Cruisers: 5000 to 10,000 tons
Destroyers: 3000 to 4000 tons
Frigates: 1100 to 3000 tons
Corvettes: 500 to 1100 tons
FAC: +25 Knots
Large PC: 100 to 500 tons
Coastal PC: -500 tons

There are, however, some nearly "standard" classifications that can be applied to most major warships. The designations listed here attempt to be "universal" to the greatest degree possible. Keep in mind that many nations chose to "do things their own way" at one time or another. A typical destroyer built during WWII and still in service and would still be classified as a destroyer, but it would be more similar to a modern frigate, corvette or offshore patrol vessel depending on its level of modernization.
In the British Navy the pendent number is written with an alphabet in front which distinguish what type ship it is.

A - Auxiliary
B - Battleship
C - Cruiser
D - Destroyer
F - Frigate
M - Minesweeper
N - Minelayer
R - Aircraft Carrier
S - Submarine
H - Hydrographic Vessel
L - Amphibious Warfare
P - Fast Patrol Boat

Cruisers: Cruisers are an extremely large and varied group.

Guided Missile Cruiser: Modern guided missile cruisers generally have only one of the traditional cruiser roles--they are either offensive or defensive, but not both. Typically offensive ships are equipped with heavy anti-ship missile batteries; defensive ships have anti-aircraft missiles to defend task forces. Neither type is truly capable of independent offensive operations as traditional cruisers were. A few guided missile cruisers have combined the offensive and defensive batteries in one large hull.

Destroyers: The destroyers have seen numerous changes in role over the years, as can be seen from the descriptions below

Definitions based on size comparison (i.e. a destroyer is larger than a frigate but smaller than a cruiser) and displacement have become meaningless in this category and the same will likely happen to the escort classes (corvettes and frigates) in the next two decades.

Destroyer: (DD/DDG(Destroyer /Guided Missile Destroyer) Small, fast, heavily armed escorts intended to protect the battle fleet, Carriers from enemy surface, air or undersea threats, depending on the era. Destroyers originated as "torpedo-boat destroyers", intended to sink enemy torpedo boats. Anti-submarine warfare was added when submarine became a threat. As aircraft became more important the destroyers added an anti-aircraft role. Destroyers are primarily defensive in nature and generally operate in groups rather than independently. In a modern context, DDs are the major anti-submarine ships and DDGs are anti-aircraft ships, but they retain multi-mission capabilities.

In addition to these defensive capabilities, it can support strike operations with long-range gunfire and land-attack missiles. It is this strike ability, mostly with Tomahawk missiles, that distinguish destroyers from frigates.

Frigates: Frigates are larger than corvettes. They are about the same size or just smaller than. Frigates today are multi-purpose platforms capable of long-range, long-endurance independent low and medium intensity missions. They are with advanced weapons, sensors and computer processing ability to defeat multiple air, surface and subsurface threats simultaneously and surviving in a high intensity environment. Stealth frigates are getting popular in the many navies.

Corvette is a small, maneuverable, lightly armed warship, originally smaller than a frigate and larger than a coastal patrol craft, although many recent designs resemble frigates in size and role.Corvette Small, generally slow escort-type vessel, generally intended for ASW. They are also sufficiently habitable and have the endurance for long-range operations. Best suited for coastal work but sometimes employed as a seagoing vessel. Often seem to be heavily armed for their size, but generally are lacking in things like sensors, electronics, reloads, range and accommodations.
Corvettes have a displacement between 540 and 2,750 long tons (and measure 180–330 feet (55–100 meters) in length. They are usually armed with medium- and small-caliber guns, surface-to-surface missiles, surface-to-air missiles, and underwater warfare weapons

FAC: Fast attack craft are smaller than corvettes and distinguishable from similar sized patrol craft by their higher speed (at least 25 knots). They are distinguishable also from inshore vessels by size, being around 50m in length, whereas the smaller craft are 30m or less. Due to their small size, FAC specialise in one discipline and cannot be considered multi-purpose platforms. Most are optimised for surface warfare using missiles and guns, and increasingly rarely, torpedoes.

Battleships: Battleships were the primary warships in the era of ship-to-ship combat with guns. They were equipped with the largest and greatest number of guns possible, and were heavily armored to protect them from similar enemy ships. Battleships were intended to engage in ship-on-ship or fleet-on-fleet combat with forces of enemy battleships. Battleship armor was generally heavy enough to protect the ship against it's own main armament

Offshore Patrol Vessel: (OPV) Modern classification for vessels similar in purpose to the 3rd class cruisers. Intended for duty in areas requiring presence but not major fighting power. Vessels tend to be optimized for good seakeeping, long range, good accommodations, easy maintenance and reliability. Generally slow and seemingly underarmed for their size

The World’s Navies classifies the ships as follows:
Cruisers: +10,000 tons
Light Cruisers: 5000 to 10,000 tons
Destroyers: 3000 to 4000 tons
Frigates: 1100 to 3000 tons
Corvettes: 500 to 1100 tons
FAC: +25 Knots
Large PC: 100 to 500 tons
Coastal PC: -500 tons