Cards are Decko's building blocks. They're like atoms of data that can be combined and connected to create all kinds of new data structures.
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Many web systems have some parallel to cards. Wordpress has "posts", Drupal has "nodes", etc. But Decko takes the atom idea much further. For example, if you have a Company card (say Widgets, Inc) that you want to attach several smaller bits of information (address, logo, contact person, etc), then each of the smaller bits of information is also a card.
In contrast, to add microstructure to a Wordpress post, you must create new custom something-or-others that themselves are not posts. The same goes for Drupal nodes. These custom something-or-others can require custom interface, custom code, and custom database tables, none of which are familiar to the rest of the system. A standard site might have half a dozen modules installed, each with custom subsystems that have no idea that the others exist. This makes data and system integration extremely difficult.
Decko's deeply atomic approach yields huge benefits. Since everything is a card, you can use the same data and interface patterns over and over again. Everything can be edited in place, without shifting context. There's just one modules api, one RESTful Web API, and one rules system. The native query language is designed for querying cards, and the views system is designed to display them.
In short, Decko just does one thing: it handles cards. But that one thing adds up to an extremely powerful yet simple management tool.