Destroyers

Destroyers are smallest ship class of 'regular' navies. Classified also as smallest escort ships, used to protect capital ships against voidcrafts and missile salvoes. Used also to escort convoys through hyperspace and as ships used to counter pirates and varierty of other, relatively small threats of space.
The last part naturally works bot hways, and destroyers tend to form main forces of majority of better organized pirate and corsair fleets. They are also oftenly used by variety of revolutionary governments and insurgents, since they can be rather easily obtained on black market. They also tend to be a sort of training ships, with new officers being assigned to them for their first missions, to fight against pirates and so on and thus prove if they have a talent for this.
Destroyers were what ultimately phased out frigates of participating in regular battles, since they were much more durable and much better armed, with frigates failing to provide necessary survivability. It is also a smallest unit to be always fitted with hyperdrives, since they are big enough for them to not pose notable change in burden.

Variants

Light Destroyer - Light Destroyers are smallest of the destroyer variants, to some point an oldest one, since they are more or less similar in size to earliest destroyers (that were firstly considered cruisers, until things changed), from which other variants evolved. After evolving, however, it was decided that there are still roles for which ships of their weight (30 000 tonnes) might be useful, thus the Light Destroyers were born. With crew around 50-60 people. They tend to be used by pirates, corsairs and patrol units that protect hyperspace routes, and that serve as pickets in uninhabited or not really important systems. They, however, tend to play secondary roles in terms of convoy escorts and AV defense.
Destroyers - Base variant of destroyers, in this times between 80 and 110 thousand tonnes of tonnage. Gold standard among destroyers, used by pretty much every star fleet in cosmos, with crew up to 150 mariners. They are used so widely simply because they are versatile, being decent in every possible role, even when not being a master of any particular.
Heavy Destroyers - Biggest destroyer variant, with tonnage around 150 000 tonnes, suprisingly resilient and heaviest in terms of weaponry from all variants of destroyers. Crewed by up to 200 mariners. Used by majority of corsair fleets of states incapable of fielding a 'proper' navy, since they have just as much firepower to obliterate orbital industry of underdeveloped planet. Not to mention being able to massacrate frigate and corvettes. They also tend to become a convoy escorts.

Configurations

Missile Destroyer - Destroyer configuration dedicated to hosting maximum possible missile firepower, capable of launching several devastating salvoes composed of SK missiles, but forced to fight in artillery range it could end up obliterated by sufficiently determined frigate.
They also tend to a sign of desperation, of sorts. Since most fleets tend to use them when desperate. They are then used in groups to assault heavier escort ships or even capital ships, in most cases by raiding secondary systems, trying to force enemy to divert his forces from main assault formations.
Escort Destroyer - Destroyer configuration dedicated to hosting maximum possible AV defense. Extraordinary in terms of defending capital ships against voidcrafts, corvettes and frigate, but practically incapable to destroy another destroyers due to its fundamental lack of heavier firepower. Their 'escort' name means escorting capital ships, not convoys.
Armored Destroyer - Destroyer config dedicated for being much better armored (by armor plates that can be glued' to armor from the outside) and heavy artillery weaponry, in most cases composed of few medium calibre MAWs capable of dealing some serious damage to destroyers, plus some weaker cannons to decimate frigates and corvettes.
They tend to be a destroyers-killer, with heavy AV defense which allows them to enter artillery range and shred every typical destroyers to pieces. Damaging (at least a bit) even light cruisers.

Power Generation

Fission Reactor

Propulsion

Rocket Engine or Ion Engine.