Destroyers (DD or DDG)
A destroyer is currently the largest type of surface combatant currently under construction for world navies. Destroyers have steadily grown in size (now 5,000 to 10,000 tons), expense (nearly US$700 million apiece) and capability. Generally,
a destroyer is considered to be a ship that has all of the sensors (including a sophisticated phased-array radar), combat systems, and weapons needed to operate in a high-threat environment. A number of world navies are currently building ships that, while called frigates, more accurately represent destroyers in size and capability. Examples include the Spanish F-100, the German F-124, and the Dutch De Zeven Provincien classes (all are highly capable ships displacing over 5,000 tons and carrying phased-array radars).
Frigates (FF or FFG)
A frigate is a medium-sized surface combatant (between 2,000 and 5,000 tons) that is either suited for one specific role (anti-submarine warfare or anti-air warfare), or has lesser all-around capabilities than a destroyer. A frigate may be less capable than a destroyer, but is still a relatively sophisticated and expensive (averaging around US$325 million apiece) platform.
A frigate is generally the smallest surface combatant that can conduct extended blue-water missions in a high-threat environment.
Corvettes (FS)
Corvettes are fast (around 25 knots or better), well-armed ships that displace between 700 and 2000 tons. A corvette is generally not intended for extended ocean-going operations, and is best suited for regional operations.
Corvettes are generally the smallest platforms capable of accommodating the sensors, weapons, and combat systems needed to operate in a medium threat environment. Corvettes are
sometimes referred to as light frigates (FFLs). It can be assumed that the hull design for a corvette and that of an offshore patrol vessel are very similar. The differences will be in propulsion and outfitting. Corvettes will have higher speed and therefore less endurance and range than OPV, much greater armament, and less space for provisions and habitability.
AMI International - NSPD Sample
At 3,144 tonnes and considering its armement and sensor suite F22P is a Frigate, comparable to e.g. the Italian Lupo/Artigliere class, though the Type 53H3 (Jiangwei2) from which she derives displaces only 2,393 tons full load and with similar armement and sensors is considered a Light Frigate by many.