A card is essentially a key/value pair. In Decko terms, every card has a name that serves as its key and content that serves as its value. The nature of that content depends on the card's type.
Both the key and the value have plenty of added meaning/power within Decko. Card names, for example, handle variants, compounds, and context, and card content has special handling for types, revisions, structure, links, and nesting.
But ultimately a card isn't a scary thing. It's just a named chunk of information .
Since everything in Decko is a card, learning how to use cards is basically the same as learning how to use Decko. You can start with a high-level introduction to Sharking or go straight to lower-level features.