The suite of the card determines what it does, and what the card is determines how strong it is. Strength from lowest highest goes 2 to Ace. The better the card, the stronger the effect it has. You also don't have a choice. For instance, if you get an Ace, you have to use an incredibly strong spell. You can't cast something weaker.
Diamonds can be used to increase defense in some way, either summoning armor or hardening skin or hardening armor you already have or summoning barriers. Et cetera. Diamonds are defense. Can also be used to cast a defensive spell of any type. (I originally had it that diamonds can summon money and rare gems but... seems kind of cheap now)
Hearts are used to heal or to charm. The healing is pretty obvious, charming you can cause someone to be attracted to anyone or anything else. You can also once again choose from existing spells that fall into the charm or heal category.
Spades can be used to summon weapons or cast attack spells. Spades are simple like that.
Clubs are buff. They make you stronger, faster, or smarter or whatever. Can be used to cast buffing spells.
In every Deck there is a Joker, which has a 100% random effect. It can literally cause anything to happen, but usually no matter what confetti or balloons appear.
You draw a hand of five cards and can't draw again until you use all five. When a deck is depleted a new one will appear either in your hand freshly shuffled. You can put the deck wherever you want, you can always summon it to your hand. So you don't have to carry around the hand you were dealt at all times.
No one can steal a deck once someone has claimed it. They must either give the deck away or be killed.
Every Deck has room for one more card that you can interchange. This spot is for the Ultimate Aces, Aces of a unique suite hidden around the land that have incredibly powerful effects. When an Ultimate Ace is added, the Deck is shuffled. If you have more than one Ultimate Ace you can add it to the deck and the current one will be removed. However you cannot exchange Aces when one is in a hand you were dealt.