Affinity is the catchall term for a magical power found throughout the Galaxy. Affinity can be spent on magical/Psychic powers that affect the game in various ways.
Any unit that has access to Affinity will have this marked on their profile, along with the powers they can spend it on. Just like Heroic Actions, some Heroes will have specific actions they can spend Affinity on, which will be specified in their unit profile special rules. In these cases, the Hero will spend the specified amount of Affinity points to enact that action. Each action will also specify what phase it takes place in. These actions can take the form of altering movements, making ranged attacks, or performing special physical attacks.
Faith is a term used to resist Affinity. Faith can represent belief in various things, whether it's the Emperor, the Eldritch Horrors, or sheer willpower to press on despite adversity.
Anytime Faith is used to resist Affinity, a unit must either be the target of the ability and spend 1 Faith point to resist the ability. The Affinity user and the Faith user will make opposed rolls. If the Faith roll exceeds the Affinity roll, the power is negated, and the Affinity user loses any Affinity points spent regardless. Destiny may be spent to alter this roll on either side. Before making the opposing Faith roll, the Faith bearer may choose to spend any amount of Faith points before they roll to add a bonus to their roll. However, they cannot spend Faith after the roll has been made.
Example: An Eldritch Hrror casts a blast upon an Empire Barge. The Empire opts to spend 2 Faith to oppose the roll: 1 Faith point is spent to resist the power, and 1 additional Faith point adds +1 to the roll. When they roll, the Human rolls a 4+1 = 5, and the Horror rolls a 5. The Empire spends 1 point of Destiny to bump his 5 into a 6, granting him the victory.
Very rarely, some Heroes will have specific actions they can spend Faith on, which will be specified in their unit profile special rules. In these cases, the Hero will spend the specified amount of Faith points to enact that action. Each action will also specify what phase it takes place in.
When an Affinity attack targets a coherent unit, the entire coherent unit may not pool their Faith. They must choose one character with Faith to oppose the Affinity attack. However, some units may have special rules that allow them to pool the Faith of units within coherency. Always check your unit profile for these rules!