Steem Monsters is a decentralized collectible card game built on the Steem blockchain. It is similar in concept to games like Magic the Gathering and Hearthstone where you build up a collection of cards, which all have various different stats and abilities, and use them to battle other players in skill-based matches.
By using blockchain technology, players can own their digital cards, and can trade and sell them freely just as if they were physical cards, without the control of any centralized company or entity.