These days, ship classifications such as "destroyer" have more to do with a particular ships role and mission than its exact tonnage and dimensions.
I was doing some research into various naval classifications for a project not so long ago. I think a lot of the general conceptions of ship classes are WWII holdovers. Frigates, smaller than destroyers, smaller than cruisers, smaller than battleships...
That's kind of the civvie interpretation, though. 'Battlecruiser' is a term that's been out of date since WWI (unless you count the Russian Kirov-class, but battlecruiser is just a nickname for them), 'Dreadnought' likewise (although you can conversely argue that all battleships built since WWI were really dreadnoughts). Battleships haven't been built since WWII. Cruisers are classically a generalist, jack-of-all-trades ship, but they've been largely removed from modern usage except as escorts (Russia being a notable exception, since they didn't go down the Nimitz supercarrier route with their navy), due to the increased power of specialists.
Nowadays, it's pretty much all destroyers and frigates, the only difference really being in size. You could make a decent argument that the only warships in service are destroyers and aircraft carriers; 'frigates' and 'cruisers' are just PR or morale-related redesignations for ASW/AA/support escort ships that hang around the aircraft carriers (again, with a few exceptions like the Russians, who use Kirovs and Kievs and the like as the core, rather than vast aircraft wings).