Cards are a convenient means of displaying content composed of different elements. They’re also well-suited for showcasing elements whose size or supported actions vary, like photos with captions of variable length.
A card collection is coplanar, or a layout of cards on the same plane.
Card content type and quantity can vary greatly. Cards within a card collection can each contain a unique data set. For example, various cards within a card collection might contain a checklist with an action, a note with an action, and a note with a photo.
Cards provide context and an entry point to more robust information and views. Don't overload cards with extraneous information or actions.