Secondary Classification (Defense)

Frigate

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Frigates are lightly armed and armored. Capable of very little offensively, often used as picket ships or in wolf-packs for strength in numbers.
Example: Oberth Class, Centaur Class, Saber Class

Destroyer

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Destroyers are usually fairly well armed, but not well armored. They pack a powerful punch when used in concert with larger attack craft or when in wolf-packs with other Destroyers and Frigates.
Example: Miranda Class, Soyuz Class, Norway Class

Cruiser

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Cruisers are usually fairly well armed and armored. If good tactics and patience are used in battle, then a Cruiser has a small chance of destroying a small Capital ship in combat.
Example: Excelsior Class, Intrepid Class, Steamrunner Class

Interceptor

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Also konwn as Escorts, (aka, Light, Heavy Escort). Interceptor's can be either lightly or heavily armed. They are always exceptionally fast, however. They are designed to intercept and slow down (or stop) potential enemies while the rest of the fleet mobilizes.

In escort duty, the same theory operates. They are designed to keep up with a convoy, and sprint ahead to meet the enemy to keep the convoy safe should the need arise.
Example: Defiant Class (Designed for Anti Borg Operations)

Attack Cruiser

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Small, well armored and armed with equivalent firepower of a Battle Cruiser or better. They are very quick and agile, yet still capable of meeting out impressive punishment and still survive. The difference between an Attack Cruiser and a Battle Cruiser is primarily speed and mission. Their entire purpose in existence is battle, they are offensive craft born and bred. Most often found on defensive lines during peace-time, and on the front lines during a war.
Example: Prometheus Class

Battle Cruiser

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The typical Federation heavy battle craft. Battle Cruisers are a good mix of powerful defenses, powerful offense and mobility.
Example: Ambassador Class, Constitution Class, Constellation Class, Nebula Class

Heavy
Battle Cruiser

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The heavy battle cruiser sacrifices mobility for power in weapons or shields... or both.
Example: Akira Class, Constitution Class Refit, Galaxy Class

Battleship

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Battleships are Exceptionally powerful, Exceptionally well armored and excessively powered for these reasons. They are capable of taking multiple craft at once, and meeting out titanic damage. Not many craft with this level of power have ever been created.
Example: Sovereign Class

Dreadnought

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A level above Battleship. A Craft holding the offensive and defensive power of a medium sized Starbase in one mobile Starship. These types of vessels are very rare, they are exceptionally expensive in terms of resources, power and manpower.
Example: Meredith Class