One of the oldest religions in history and in Ancient times the only major religion in the Middle East other than Judaism and polytheism. Its doctrine centers around an eternal war between Good (represented by Ahura Mazda) and Evil (represented by Angra Mainyu, or Ahriman). The former can be roughly equated with God but the latter is definitely not Satan, since Satan is a rebel angel/spirit/former prosecutor fighting his own creator (beginning open rebellion around the time the Persians were running things, for some reason) - thus, Failure Is the Only Option. Ahriman is Ahura Mazda's uncreated Evil Counterpart and thus an equal or near-equal God of Evil who actually has a shot at the title (though akin to Satan, he will lose in the end anyway). Humans have free will, and they face their own personal version of the divine struggle, between Truth and the Lie. Based on one's actions in their struggle, they are either led to paradise (the word comes to us from Zoroastrianism) or hell. Interestingly, hell in Zoroastrianism is not eternal, but meant to reform people.
The religion was originated by a prophet called Zarathustra (Anglicized to Zoroaster, via Greek Zoroastres). Historians aren't quite sure when he lived, with estimates ranging from c. 1000 BC to c. 600 BC, tending towards the earlier estimates. He is thought to have lived in what is now northern Afghanistan, though, again, historians are not quite sure. Zoroaster wrote the Gathas and seven of the Yasnas, parts of the text known as the Avesta, which is analogous to The Bible.
Zoroastrianism was the national religion of the various pre-Islamic Persian dynasties. After the Islamic conquest of Persia they were sometimes regarded as "honorary people of the book" because they were so well established in their country, and they lived in a second class but tolerable status alongside Christians and Jews. Nonetheless several of them have fled to India where they remain today as the "Parsees". The Parsees were favored in the days of The Raj and had something of a reputation as a Proud Merchant Race.
Among their more interesting customs was the belief in the sacredness of earth, water, and most famously, fire. This led to the custom of keeping an eternal flame at Zoroastrian temples. This also led to the mistaken belief by outsiders that they actually worshiped fire rather then regarding it as a symbol of Ahura Mazda. But one of the stranger aspects of this was the custom of disposing of the dead by placing them in raised platforms ("Towers of Silence") for the birds to eat to avoid desecration of sacred elements.
Though now a small religion, Zoroastrians can be found in a number of places in the world. The largest groups in Asia are the Iranian Zoroastrians and the Parsees of India. Probably the best known Zoroastrian in the world was one Farrokh Bulsara, a British Parsee everyone knows as Freddie Mercury.