Card System
In Ragnarok Online, there are items that have suffix of "Card" on them, with prefix being monster's name - such as Poring Card. These cards are installed into equipments to provide specific bonuses to your character! For a list of cards, see the category of cards.
Acquiring Cards
You can acquire cards in different ways:
Many monsters in the game have small chance of dropping a card featuring themselves on it. You can get one from an Old Card Album. Card you get from it is random. You can trade or buy one from a Merchant's vending shop.
Installing Cards
By double-clicking the card, you can "compound" or 'install' the card into a piece of equipment of your choice. Click cancel if you didn't mean to do it. Card does not disappear from inventory until you actually install the card to an equipment.
To install, the equipment must have a 'slot' available for a card to go in. You can't just install it to any equipment you want. Also, cards only install into specific type of equipment set - but this is evident since when you double-click the card to install, you are shown list of equipments you currently have that you can put the card in. You obviously don't see anything if you don't have any eligible equipment.
Right click the card item to see what effect it gives, and where it is installed.
To see if your equipment has a slot available for a card, simply mouseover the equipment in question. if it does, it'll appear with suffix [1], [2], and such. An example would be 'Guard [1]', which means it has 1 slot for inserting a card.
Only weapons have more than 1 spot for installing cards.
Caution
Once you install a card into an equipment, you cannot remove the card from equipment under any circumstance! Knowing that, please think again if you truly wish to install the card into an equipment - especially if equipment / card you are compounding are rare or expensive.
Also note, if equipment is destroyed because you failed when you tried to upgrade it, equipment AND all of the installed cards will be destroyed. So it is recommended you get an equipment already upgraded to highest level you can get (either by upgrading by yourself or buying one from other players) before installing the card.
The above does not affect items that are merely broken through enemy or player abilities. All cards, upgrades, and such are still intact on a broken equipment.
Card Types
These are list of equipment spots cards may be installed to.