There are four different types of damage that can be applied to and resisted by a player's ship:
EM Thermal Kinetic Explosive
The damage types done by a weapon are determined by the ammunition loaded into it, with some ammunition being capable of doing more than one damage type. The exact composition of the damage is listed within the item information of the ammunition.
Each damage type will be resisted against individually by the respective resistance type of the ship being shot at. A shot doing only Thermal damage will do full damage against a ship with no thermal resistance, even if its Kinetic Damage resistance is very high. A shot causing Thermal and Kinetic damage on the same ship will accordingly only have the Kinetic damage part of the damage composition reduced by the resistance.
Polarized weapons have the intended side effect of removing all damage resistance of the ship they are equipped on. This also negates the resistance increase from modules and rigs which may also be equipped.