If you haven't heard of The Witcher 3's GWENT card game then this is the video you need to watch.
Gwent is a collectible card game you can play in The Witcher 3: Wild Hunt. It puts you in a role of a general battling an enemy army with your own customized deck of cards. These cards can be obtained throughout the game world: bought at shops, acquired from NPCs after winning gwent matches, or won in high-stakes tournament in Novigrad.