Dualism is the belief in the existence of two opposing divine (or semi-divine) forces.
In Zoroastrianism and Abrahamic faiths these tend to be two opposing forces in conflict, that is, good and evil, usually a Creator and a destructive antagonist. This sort of dualism is also called ditheism.
In Eastern and Nature-based dualist beliefs, these opposing forces are more likely to co-exist peacefully or work in harmony together, the opposites complimenting one another. As in a Father God and a Mother Goddess. This is also called bitheism.
The word Dualism also applies to the philosophy that the world is made up of opposites. Night/Day, Light/Dark, Black/White, Good/Evil, Male/Female, Hard/Soft, Wet/Dry, Passive/Active.