A definition of a card (on the web) by Khoi Vinh:
A card is a single unit of content or functionality, presented in a concise visual package. More advanced cards use that form to surface content or functionality from other apps, and allow users to interact with that content or functionality directly in the context of where a user encounters the card.
Think embeddable tweets for example. Speaking of: those Twitter Cards just got taken to the next level as it’s now possible to make a purchase directly from within the card (pictured above)!