Plasma cannons are main weapons used by T1 and above ships. They deal EM damage by default and are most commonly used by Interceptors as a shield smasher.
As they deal EM damage, they are more effective against shields rather than hull.
Plasma cannons deal EM damage which is directly lethal to shields, thus making them an ample first strike weapon against ships.
Damage, like every other weapon, increases the higher grade the weapon is.
Plasma cannons can utilize weapon mods in T2 ships and above. (Include modifiers here)
Both systems are commonly used together to enable two types of damage on a single weapon.
Plasma Cannon: Interceptor Only
Longest range interceptor gun, and becomes more accurate the longer the fire button is held. Average damage for an interceptor weapon.
Rapid Fire Blaster: Interceptor Only
Short-range, rapid firing plasma cannon. This weapon gets less accurate over time, so it is more effective as a close range weapon.
Singularity Cannon: Fighter Only
Fires slow globules of plasma that deal large amounts of EM damage. Travel through enemy ships, so they can damage multiple enemies, and they deal damage over an area. Short to medium range. Due to slow projectile speed, it can be hard to use against interceptors, but this weapon is extremely useful for PVE missions.
Positron Cannon: Frigate Only
High speed, long range plasma projectiles. Fairly accurate, low rate of fire however. They are chargeable, and become more accurate after charging. Overall they have the lowest DPS rate of the Frigate weapons and it can be quite difficult to hit fast moving targets at the weapon's maximum range.