The CORE deck contains all the "core rules" of the game: player agendas, character cards, conditions, and random oracles.
This deck alone doesn't have specific flavor, but it can be used to build any game you want to play.
The Encounter deck is for building confrontations with challenging creatures or situations. By dealing two random Encounter cards and an Emotion card, you create a unique challenge.
Cards in this deck tend to be flavored for a fantasy adventure game. However, you can make cards tailored for any other setting.
The City deck contains cards for creating a city full of excitement and adventure. Cities are defined as neighborhoods connected by power dynamics, and represented by strong personalities.