Personal experience of card system. Results from google are a bit outdated because it shows the old card system. For example, OLD SYSTEM: combining 3 normal poring card will give you 1 silver poring card. NEW SYSTEM: The card system has been revamped into combining 5 different cards to get 1 random card of higher rank.
What is cards?
Cards give status boost to a character when equipped. Higher rank cards will give more boost.
How to use cards?
Drag the card into your character window (shortcut key:C) More cards slots are unlocked as you level up.
Where to obtain cards?
Kill monsters and pray hard they drop one(usually green card). Higher ranks can be obtained through synthesize and occasionally boss/dungeon drops. Cards obtainable can be checked in the card album. Credits: Flash2k6
Card Ranks. Need to be confirmed
There are 3 ranks in accordance to the color of the card frame. Green (Normal and Normal+), Blue (Silver), Yellow (Gold), Source: RO2 website. Higher ranks card are obtainable through combining 5 cards of the same rank. Read ?results from combining cards? for more info.
What is card sets?
Card sets are combination of certain cards equipped in card slots. In return, it will give extra stats boost(read card description).
How to combine cards?(only tested twice and need more confirmation. However, simple steps are provided)
Talk to card npc and buy 1 card refine stone from npc. Prepare 5 cards(must be 5 different cards). Talk to npc and select ?combine? option. Place 5 cards and stone inside (I don?t know what is the extra slot for but I am guessing cash items to boost the success). Press combine and pray hard it is successful as it has 50% of success rate only.
Results from combining cards (Need confirmation as well) Special thanks to Flash2k6, Dztnhy, Hikyuu for helping.
5 different cards combined together to produce a higher ranking. In regards to higher ranking, for example, 5 different normal card = 1 normal+ or 5 different normal card = 1 blue. I don?t know how it works but I am guessing that it is random or certain requirements have to be met(level of card). The combination of 5 green cards may result in either normal + or blue.
Apparently, the card obtained from combination is one of the original 5 cards you used. The level of the card produced seems to be the card(used in combination) with the lowest level.Different input will affect its result though I don?t have enough data on that yet.
How to replace cards?
Just drag a new card over the card you want to replace. Warning: Old card will be destroyed