This conflict takes place in the human heart and mind, not in the material Universe. It is the constant struggle between good and evil in human beings. This is ethical dualism, the dualism of Good and Evil. In later traditions this changed into a dualism that took in the material world, dividing the Universe into two camps, each ruled by the Good God or the Evil Spirit. The teachings of the Gathas, the original work of the Prophet, tend toward ethical dualism.
Zoroastrian worship involves prayers and symbolic ceremonies said before a sacred fire.
This fire, which was a God - symbol even before Zarathushtra, was used by the Prophet and by his followers ever after as the ideal sign of God, who is light, warmth, energy. Zoroastrians do NOT worship fire, as some people believe. They use Fire as a symbol, or an icon, the focus of their worship. Zoroastrianism does not teach or believe in reincarnation or karma. Zoroastrians believe that after life on earth, the human soul is judged by God as to whether it did more good or evil in its life.