Affinity is a feature which affects any card from the Renegade edition and beyond, enabling them to have slightly different traits and thus creating two types of the same card.
Affinities effectively modify some aspect of a card to make it align more towards one element, adding increased strategic value to a card that would normally be more mundane.
A prime example is the Nomad which can have either a red or green affinity, which alters its Company passive.
Affinities are not set in stone on their effects however, as some affinities provide effects that don't make sense with their actual element.
Affinities come in four types:
Fire Affinities typically benefit a card in some offensive manner, usually by increasing damage output or providing some damage-boosting effect.
Frost Affinities typically benefit a card in a defensive manner, typically by providing damage reduction effects or shields.
Nature Affinities typically benefit a card in a restorative or controlling manner, often by providing some form of healing or crowd control effects.
Shadow Affinities typically provide a card with a risk/reward trait, often in the form of making a sacrifice or taking a risk to get a much stronger effect.