Arcova Scout Frigate
None of the scout frigates are exciting - they are used to scout, and that's about it. The Arocva does have a bit of an ace up its sleeve however, and that is a researchable special ability called Timed Explosives. With this ability, the Arcova can attach a powerful explosive to a structure that does 1200 damage. Three Arcova are enough to take out many structures. The Timed Demolition rush has been used with some effect, as the Arcova is easy to slip behind enemy lines and are rarely seen as a combat threat. Pulling it off does require that you be good at micro-management however, because the Arcova is weak and will explode at the slightest hint of danger.
Cobalt Light Frigate
The Cobalt isn't a bad ship, really. It has a respectable amount of hitpoints and shielding. It is fast, useful against most targets, and cheap to build. All of these traits seem to add up to a good unit, but ultimately the Cobalt is overshadowed by the Javelis, which is just as cheap and packs a ton of firepower. Despite being a basic unit, the Cobalt actually comes into its own in the late game, when missile frigates are more easily countered. The Cobalt's special ability - Sabotage Reactor - also becomes useful, as it disables the abilities of the target for a limited period of time.
Javelis LRM Frigate
A zergling for space. I'm not kidding. The Javelis is considered to be a bread-and-butter unit for early-gaming rushing. It is cheap, it is easy to research, it has long range and a lot of firepower, and the units most suited for countering missile frigates don't become available until later in the game. Once fighters become available and cruisers start to become common, the Javelis does lose a bit of its edge. But even in the late game, it is considered a staple unit, as large groups of missile frigates put out a ton of damage. The Javelis is particularly effective in large battles thanks to the Cluster Warhead ability, which gives the Javelis splash damage when activated.
Krosov Siege Frigate
It's a siege frigate. It bombs planets. It isn't much different from the siege frigates the other factions have, although it is notable for having a very useless ability, known as Heavy Fallout. I believe that as of the release of the Entrenchment expansion, this ability - which reduces planet growth by 30% - still has the unfortunate drawback of remaining in effect even after you capture a planet.
Garda Flak Frigate
The Garda is the resident anti-strikecraft unit, built to ward off enemy fighters and bomber. It fulfill its role very, very well - only a couple Gardas are required to knock a dozen strike craft out of commission before they cause much damage, which is impressive, considering how must carriers cost. Even better, the Garda is impressively well armored and has a large number of hitpoints, which makes it a good tank unit. Some people use them as a counter to missile frigates because of this - in a straight up fight, the Garda can shrug of the small missiles, which are better suited at taking down lightly armored targets.