Set within the Warcraft universe, Hearthstone is a digital-only, turn-based collectible card game which pits two opponents against each other. Players select a hero from one of ten classes. All classes have unique cards and abilities, known as hero powers, which help define class archetypes.[4] Each player uses a deck of cards from their collection with the end goal being to reduce the opponent's health to zero. There are four different types of cards: minions, spells, weapons, and hero cards. Quests are a specific type of spell only found in three expansions.[5][6] These cards are ordered by rarity, with Legendary cards being the rarest, followed by Epic, Rare, Common, and Basic.[7] Blizzard releases expansions of additional cards every four months to increase the variety in the metagame. The game uses a freemium model of revenue, meaning players can play for free or pay to acquire additional card packs or content.