There are basically two types of cards in the game and are classified as Unit Cards (characters/fighters), and Spell Cards (buffs/de-buffs and the like).
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UNIT CARDS
Unit cards are the very characters that you get to summon in the main game board which mainly depicts your available fighters/defenders currently on play and should remain there unless defeated or removed. Units have two stats:
Strength (unit on the shield icon) - depicts how much damage the Unit Card will be able to deal/receive when it is attacking/defending.
Health (unit on the heart icon) - depicts how much damage it could take (beyond its strength) before it is defeated and removed from play. If this number reaches zero, then this card will be destroyed. Some Unit cards can also have special abilities, or useful add-on effects when properly paired with activating cards, events, characters in the game. For instance the Feral Squirrel gains 1 Strength whenever Rend (or a Spell card) is used.
SPELL CARDS
Spell cards will release a special ability when it is called upon for use in the game (which will then remove the said card on play). There are a variety of Spell cards to earn and use in the game ranging from those that buffs your units by making a them stronger or deal more damage, those that lets you draw more cards from you Deck, de-buffing your enemy or his/her cards on play, or even returning a card from the battlefield to your hand.
All cards that you wish to put into play have a Gold Cost (unit on the coin icon) which shows how much Gold the player must have or pay before summoning a Unit or gaining a spell card's effect.