To go further than Nyaa: it's a 2D side-scroller where enemies and chests randomly drop cards of various levels, types, and rarities. The cards are used to modify your special attack, and up to 4 can be inserted. Some, as Nyaa said, can have regular effects (and will chain if you set a shield+healing as an example) while other cards are more passive and instead modify how strong and how much MP special consumes, or how long some effects last.