Each card in the game has a rarity that indicates how likely it is to be found. Cards that are more rare are also more expensive to purchase in the Card Shop, but are worth more gold to sell.
There are four card rarities in Lost Portal:
Common cards are the most easily found cards in the game and therefore cost the least.
Uncommon cards are more difficult to find and therefore are worth more than commons.
Rare cards are expensive and difficult to find, usually only found after defeating a boss, purchased in the Card Shop or created in the Forge.
Ultra-rares are the most difficult to find cards in the game. There is a chance when you defeat a boss of finding an ultra-rare instead of a rare.
Avatar cards can only be found by defeating enemies. They are not available in card shops or forges.