There are three different types of ship classes, and they all have their different uses and weapons, such as frigates have turbo shields, cruisers have heavy plasma cannons, etc.
The heavy yet powerful ships in the fleet, cruisers are in charge of the heavy weapons in the fleet, as well as shields, armor, everything. But despite their superior firepower and ability to take a lot of damage, they are vulnerable to frigate spam (if they are equipped with anti-cruiser weapons) and fighters equipped with rockets or torpedoes. But cruisers have their backup; fighters and frigates are allowed to escort cruisers.
Frigates are usually complained about by people wanting them to be more powerful, seeing as their Turbo Shields fail the frigate captains very quickly. There are other reasons, like not enough hardpoints, not enough bonuses, too expensive, etc. But they are good support units. Two or three of them escorting a cruiser would probably mean that cruiser lasts until the end of time... unless fighters are involved. Fighters can kill a frigate extremely fast, even if the frigate has anti-fighter missiles equipped. Against cruisers, however, and obviously other frigates, frigates can be very powerful together. in a big group, they're no match if they have a fighter escort.
The fast hitters of the fleet, fighters are the only units that come in groups of up to 16. Fighters don't seem powerful at first, but with correct arming, strategy, and placing, they can drop an enemy cruiser in seconds, as long as there's a lot of them. Fighters are the fastest ships in the game, and versus a frigate, are extremely powerful, even alone.