The most common type of special is weapon boosts. Weapon boosts enhance weapons of a given damage type. For example, equipping Focused Optics I increases the range of all Energy weapons by 250 m. Note that you cannot use multiple weapon boosts with the same effect(s). As an example, you cannot equip both Phased Projectiles and Phased Magnets because they both increase the shield bypass of Projectile weapons.
Squadron boosts are very similar to weapon boosts, but they affect squadrons instead. They are not bound to any damage type, allowing them to enhance all squadrons.
Thrusters increase a ship's agility. For instance, Rotation Thrusters increase a ship's rotation speed. You are not allowed to use multiple thrusters on the same ship.
Cargo holds increases a ship's cargo capacity. They are perfect for hauling resources from defeated NPC Targets and player bases.
While most carriers have a built-in Support Field, T6 carriers and above may be able to use field generators - yet another type of special - to project a customized field of effect. You cannot use multiple field generators on the same ship.
Utility specials include specials that do not fit in any of the above categories.