Zoroastrianism is the religion and philosophy based on the teachings ascribed to the prophet Zoroaster (Zarathustra, Zartosht). Mazdaism is the religion that acknowledges the divine authority of Ahura Mazda, proclaimed by Zoroaster to be the one uncreated Creator of all (God).
As demonstrated by Zoroastrian creed and articles of faith, the two terms are effectively synonymous. In a declaration of the creed — the Fravarānē — the adherent states: “…I profess myself a devotee of Mazda, a follower of Zarathustra.” (Yasna 12.2, 12.8)
Zoroastrianism was once the dominant religion of much of Western- and Central Asia but as of 2007 practiced only by fewer than 200,000 worldwide, with its largest center in India. (courtesy wikipedia)