From http://forum.keypublishing.co.uk/showthread.php?t=35266&page=1
Q: Whats the difference between a Frigate, Destroyer, Cruiser, Corvette, and whatever, what are their definitions?
A: Size and the amount of armament on board is the difference.
Roughly speaking the size regions seem to be:
Corvette 700-2,000t
Frigate 2,000-5,000t
Destroyer 5,000-10,000t
Cruiser 10,000t+
Seems that the original role distinctions have blurred over time. However it is interesting to know the original distinctions.
I found this post by Quarto:
Keep in mind that, originally at least, destroyers were not anti-capship vessels. The name 'destroyer' is a highly
misleading one, because it's a shortened version of their original name - 'torpedo boat destroyer'.
All things considered, it's pretty unusual for destroyers to be operating alone, and even more unusual for them to
undertake offensive actions alone. As such, they don't need strike fighters - nor do they need large escorts,
because they already are escorts :p. Cruisers, on the other hand, are usually armed with more anti-capship weapons
(both the Plunkett and the Hades class has anti-capship plasma weaponry), so they also don't really need strike
fighters - although the Hades class certainly has them anyway.
From reading a lot of opinions around some points of agreement seem to be:
Corvette: Small (80m) torpedo ship that takes on capships.
Frigate: Meium (100-200m) Have missiles coming out of the airlocks.
Destroyer: (300-600m) Medium-large ship that picks on the little guy.
Cruiser: Big MF (1200m+); ship to ship combat and planet bombardment.
Battlecruiser: Weak armor and shield but fast and overly well armed. Kills or flees.
In other words, when a corvette is coming after your cruiser you stop it with a destroyer, then a cruiser comes to
get your destroyer you torpedo it with a corvette. Frigates just thicken the space with missiles and hell with it.
