The Binding of Isaac: Four souls (or just "Four Souls") is the official TBoI card game. The game is for 2 players or more, and is about deceiving others, kill each other, looting, gaining items, and winning souls.
There is an official website here, which shows everything you need to know:
Winning[ | ]
Just like the title of the game says, you need four souls to win, and gaining souls is usually by killing Bosses. But there are other ways to obtain souls. There will be bonus souls, that are tasks you need to do to gain those bonus souls. Another way is by gaining the 'lost soul' trinket, which is in the loot deck. An additional way to gain souls is through gaining some treasures, some treasures may give you souls, but there is only a couple of treasures that do. The final way of gaining a souls is by playing as the 
The Lost.
Treasures[ | ]
Treasures, or items, are obtained by paying 10¢, if you gain it this way you may purchase a item from the shop, or from the top card of the treasure deck, which is risky. The shop starts with 2 items in it, and can be expanded with event cards, and other treasures.
Loot[ | ]
Loot cards are essential to play, since it gives you money, bombs, and other goods. This deck is where you can find 'Lost Soul'. At the start of your turn you always loot one.
Monsters[ | ]
Monster cards are things you can always attack. Monsters have hp, a dice roll you need to roll or higher, and damage. Monsters usually have a reward for killing it, like loot, money, treasures, and bosses always give a soul, or 2 souls! At the start of that game, there are 2 monster slots, and can be expanded by event cards, or sometimes treasures. There are many types of monsters, the most often cards from the monster deck is regular monsters, and boss monsters, but there are sometimes event cards which do special things, like skipping turns, gaining loot, and rarely a soul. Monsters sometimes have effect text like if you roll a 1 take double damage, or when this dies kill a player. Players only may attack once, and may attack the top deck of the monster deck for a gamble.
Room Deck[ | ]
The room deck is an optional deck you can play with, which gives players an extra challenge, and another thing to pay attention to. Every time a Monster dies, the active player may discard the current room deck. The room deck holds cards that helps every one, hurts only one person, and rarely a monster (there is only 1 monster out of all 50 cards)!