Cardverse is an AU where the characters are in positions like what you'd find in a deck of cards. The Hearts kingdom, as most authors and artists choose to call the suits, have Italy as the Jack, Japan as Queen and Germany as King. The Diamonds are Switzerland as the Jack, Liech as the Queen, and France as King. The Spades have China as Jack, England as Queen and America as King. The Clubs (or Cloves as a rare few people say) are Austria as Jack, Hungary as Queen and Russia as King. Jokers are Prussia and Sealand. Many people choose to focus cardverse fics on the Spades kingdom.
Very little is cannon in cardverse - just the positions and outfits. Authors and artists can create their own views of what the cardverse AU should be. Most stories centered around cardverse portray it as at least somewhat medieval, and many include magic as a normal or at least a not-unheard-of part of life.
It is commonly acknowledged that the titles of King, Queen and Jack (and Ace in the fics that include them) are merely political and hold no connection to the gender of the person in that position. Kings can be female, Queens can be (and are) male, and there is (usually) no need for King and Queen to be married, although they often are portrayed as such.
In most stories, the King is in charge of the Military, the Queen has the Navy and finances, and the Jack helps with anything else needed. In the fanfics that include Aces, their identities usually are kept a secret from everyone but the other monarchs in their kingdom. In some of these stories they are also shown as spies that gauge the unrest or contentment in their kingdom and report to the monarchs any disturbances in the kingdom or possible rebellions on the rise, all while maintaining the cover of a regular civilian without any significance.
Another commonly acknowledged fact of cardverse, though not always, is that there are many wars and battles and that Spades and Clubs do not get along well, more than the others.