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. 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. These cards are ordered by rarity, with Legendary cards being the rarest, followed by Epic, Rare, Common, and Basic. 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.