I'll try to answer this without giving away too many spoilers (I'm on book 9).
The Deck of Dragons is a representation of the warrens and their dimensions, in the form of Houses, where the pantheon of ascendants and gods (and their servants) would be allocated cards, representing them. For instance,
the king of High house death would be Hood, because he is the ruler of that warren/dimension.
The cards are used in the world of the books to "read" the occurrences (be it past, present or future) which are happening within the warrens or throughout the world, usually linked to each other due to important events or movements of powerful entities.
A quote from Quick Ben:
"...the High Houses of the Deck relate to certain warrens and as such they present a kind of window looking in on those warrens—conversely, of course, things can in turn look out from the other side, which is what makes a reading so . . . risky. The Deck is indifferent to barriers—in the right hands it can reveal patterns and relationships hidden to mortal eyes." [excerpt from Dust of Dreams].
What he implies is that the deck (and the reader) is endowed with the ability to look into the dimensions of the warrens, and you'll notice throughout the books that readings are done to either ascertain current events and movements of the pantheon, or to determine how events will unfold in the future (prediction).
Also, the individuals who do the "readings" use the power of the warrens to be able to do the readings, for example: