It was the duty of the believer to side with Ahuramazda, which was possible by avoiding lies and bringing certain bloodless sacrifices in the open air. (Small towers, containing the holy fire that was needed for these sacrifices, can still be seen.) At the end of times, the believers will be judged by Spenta Manyu; those who have never lied, will go to heaven.
The winged figure known as Faravahar (picture) is not Ahuramazda himself, but a symbolic representation, which reminds the believer that his soul must progress towards god. Ahuramazdism was the first religion that made ethical demands; its holy book is the Avesta. It is not certain that the kings of ancient Persia -like Darius and Xerxes- were adherents.