Taken from the game's introduction
Faces is a role playing game where the players play as the deepest part of a person, the most secret and unknown place in someone’s psychology: their subconscious. All but one of the players each take on a part of the subconscious, known as a Face, while the final player plays as the person themselves. The game is about looking at and exploring a person, what they think, what they don’t think and what they do because of what goes on inside their head, even if they don’t notice it.
One player plays the Mask, the person itself. The other players each play one face.
Each face has five rated traits: Dominance, Pervasiveness, Id, Absorption, and Expression. It also has two Id aspects, selected from the categories Memory, Emotions, Instincts, and Tendency. The aspects define what the focus of the face is. For example a face may focus on a Childhood memory combined with the Hunger instinct. Or on the emotion of guilt with the tendency to anticipate. A face also has a synapse pool that determines the action points of that face.
The mask (whole person) also has rated traits: Consciousness, Strength, Guile, and Intelligence.
The mask player starts with a beginning incident that should be the beginning of some kind of conflict. The game mechanics revolve around absorbing facts by the faces and reacting to them by trying to influence the mask. The mask can introduce new aspects from its surroundings, but it is driven by its different aspects. Thus the players drive the story together and explore the character of the mask.