Cards are rare Item dropped by monster in Ragnarok Online. The chance to get a card from a monster is 0,01%.
Every Card has a picture of the monster which dropped the card. Every Card has a different effect.
To use the effect of the card you have to combine the card with your Weapon or Equipment.
Some Weapons and Equipment got "Slots".
You can can combine Cards with Weapons/Equipments only when they have a slot. When you use a card the name of the Weapon/Equipment change a bit.
An example a Mantle is combined with a Poring Card and the name is now Lucky Mantle. Choose wise which card and Equipment you use the card can not be
removed from the Equipment.
For combine a card with your Weapon/Equipment both item have to be in your equipment.
Double Click the card, a menu pop up. In that menu you can choose the Weapon/Equipment to combine Weapon/equipment with the card.
After this process the slot of the Equipment/Weapon is filled with the card. So it isn't possible to put a new card on this slot.