Imperial Navy Escort
Escorts are the smallest armed vessels available to the Imperial Navy. Lightly-armed and armored, with barely any shields, these ships are extremely fast to compensate.
Incredibly common in fleets, these ships are normally assigned sector patrol duties or perform as escorts for much larger ships. In fleet engagements they will also act as scouts, ranging out in front of their armada to verify enemy ship numbers and locations.
In Battlefleet Gothic: Armada 1 & 2
All escorts come with the Sensor Sweep ability, which allows a player to ping nearby enemies and keep them identified and targetable for half a minute, even if the target would normally enter stealth.
Destroyers
Cobra
Fast and slight, the Cobra is a dedicated torpedo platform. Armed only with a small Macro turret and two Torpedo tubes, a single Cobra is incredibly vulnerable on its own. As part of a squadron of three however, those torpedos suddenly become a threat even for slower and larger ships. Just don't allow them to hit you back.
Widowmaker
A Cobra variant, the Widowmaker drops one of the macro turrets for an enhanced sensor suite that increases its sensor range immensely. In this regard, it can serve as a long-distance spotter for other ships, and with its small profile and high speeds, allows it to detect enemies, then zip away out of danger without being seen in return.
Frigates
Sword
Common and reliable, the Sword-class is also armed simply, with two laser macro-batteries to do the Emperor's work. Although much bigger than the Cobra, it does the same sort of fleet duties, such as being outriders of bigger ships, or deterrents to incoming fighters or torpedoes.
Falchion
A somewhat newer class, the Falchion was designed primarily to protect larger ships from enemy escorts. In this role, it uses Torpedoes and laser macro-turret to chase off any incoming attackers. Mostly constructed by the Forge World Voss Prime.
Firestorm
A modified Sword-class, the Firestorm trades one of its laser turrets for a prow-mounted Lance battery. The latter weapon gives the Firestorm options for hurting bigger, and much heavily-armored ships.
In Battlefleet Gothic: Armada, the Firestorm is ironically more suited to hitting smaller and faster ships at long range (due to the fact that its Lance battery never can't miss) over taking potshots at Battleships or Cruisers. Go figure.
Others
Claymore
Essentially a Sword class used by the private sector, the Claymore is used to protect troop transports and cargo haulers from opportunistic raiders. Extremely common on the spaceways, due to how easy and cheap they are to construct and maintain.
Turbulent
A "heavy" Frigate, with armor comparable to Cruisers. Used primarily to range ahead of fleets and eliminate any enemy scouting force they encounter, the class has gained a reputation for being lucky. Its standard comms equipment is somewhat antiquated however, made for durability rather than user-friendliness.
Tempest
A heavy Frigate configured for close-ranged action, with shorter-ranged broadside batteries and Assault Boats for boarding actions.