Zoroastrians were (and still are to a small extent) followers of the philosophy and religious teachings of a semi-legendary man named Zoroaster or Zarathustra. This religion originated in Persia in the 6th century BCE.
Zoroastrianism was one of the early monotheistic religions. Zoroaster taught that there was only one god, a deity named Ahura Mazda. Ahura Mazda was the source of all that was good. However, he was also opposed in the universe by evil. Human beings were given the right and the ability to choose to follow good or to follow evil. At some point, it was believed that the world would end and everyone would be judged based on the choices they had made.
These beliefs are, of course, very similar to some Christian beliefs and some scholars believe that Zoroastrian beliefs influenced Christianity.