Proto Indo-European: An extinct ancient language which scholars have partially reconstructed from living languages that are branches of the Indo-European language family (e.g. English, French, Spanish, German, Greek, Norwegian, Pharsi (Persian), and Hindi).

Proto Indo-Europeans: An ancient tribe, consisting of many sub tribes, which lived, before 4,000 B.C. as one people on the Russian steppes to the east of the Volga river.  Sometime after 4,000 B.C. certain sub-tribes of this vast tribe began to migrate. Some migrated West into Europe to become the ancestors of most present day Europeans, while others (the Proto Indo-Iranians) migrated south to Iran and India, conquering the peoples who already inhabited these lands.

Proto Indo-Iranians: An ancient sub-tribe of the great Indo-European tribe, the proto Indo-Iranians lived as one people, sharing a common language culture and religion, until around 3000 B.C. on the steppes of Russia by the Volga river.

Aryan: What the Proto Indo-Iranians called themselves.  What "Aryan" originally meant is unknown, but it later came to mean simply "gentleman" in India.  "Iran" means "the Kingdom of the Aryans".

The Vedas: The Sacred writings of the Hindus, the Vedas were written over many centuries by various authors.  The Vedas consists of three major parts, the Samhitas, or collection of verses (mantras), the Brahmanas, collections of rituals and sacrificial ceremonies which were performs by the inherited class of priests (Brahmins), and the Upanishads, collections of discourses concerning the ground of all reality and the inner essence of the human being.

The Rig Veda: The oldest and most important of the four Samhitas, the Rig Veda consists of hymns in praise of various deities worshipped by the ancient Indo-Iranians.  The oldest of these are believed by scholars to reflect the religious ideas of the Proto Indo-Iranians when they lived in their ancient Russian homeland.

The Zand Avesta: The Sacred writings of the Zoroastrians, the Zand Avesta, like the Vedas and the Bible was written over many centuries by various authors, but scholars believe that the most sacred of these, the Gathas, were written by the prophet Zoroaster himself.  The oldest portions of the Zand Avesta contain religious beliefs very similar to the oldest portions of the Rig Veda and are believed by scholars to reflect the religion of the Proto Indo-Iranians.

Zoroaster: The founder of Zoroastrianism, the pre-Moslem religion of Iran, Zoroaster lived in the east of Iran sometime between 1, 00 and 1,200 B.C.  Zoroaster was a monotheist (perhaps the first monotheist) who believed that God (whom he named Ahura Mazda, "Wise Lord") spoke to him.  Zoroaster did not think he was founding a new religion but was restoring to purity the ancient religion of his people (i.e. the ancient religion of the Indo-Iranians).  All present day scholars believe that Zoroaster did more than that and that, from hints contained in the ancient religion, he transformed it from being a fairly ethical type of polytheism into a lofty ethical monotheism.  Zoroaster apparently believed that the ancient  sky god Varuna was the sole uncaused and eternal deity who created all the other gods (which in Zoroaster's thought might be better conceived of as arch-angels), the material work, plants, animals, and human beings from nothing. He did not refer to Varuna by his ancient personal name, however, but called him simply "Wise Lord." Zoroaster believed in the existence an original evil spirit (Angra Mainyu), the cause of all evil and death, who continually attacks God and his creation but will be defeated in the end. Zoraster believed that the ancient God Indra was no god at all, but one of the demon spawn of Angra Mainyu. 

Asura (Ahura) (lit. "Lord"): A name applied especially to certain gods by the Indo-Iranians (e.g. Varuna). The Asuras were personifications of natural forces and/or abstract concepts.  Though personal beings they lacked the rather human flaws and foibles of the devas (deavas) and were characterized by their highly moral nature.

Deva (Daeva) (lit. "shining one"): A name applied especially to certain gods by the Indo-Iranians.  Unlike the asuras, the devas had recognizably human characteristics and flaws.  This is especially true of Indra, the Indo-Iranian war god who was the greatest of the devas. Largely because he disapproved of Indra, Zoroaster denounced all the devas as devils and forbade their worship.

Varuna: A sky god of the Proto Indo-Iranians, Varuna seems to have originally been the greatest of all the gods. Ancient religious texts describe him as omniscient, knowing the hearts of all.  A very moral deity, he was thought to uphold Rita, to punish wrongdoers, and reward the righteous.

Indra: A proto Indo-Iranian god of storms and war.  After the Indo-Iranians moved east and south into Iran and India, Indra became the head god of the Indo-Iranian religion, replacing Varuna.  When this occurred he took on some of Varuna's characteristics, but never wholly lost his violent and rather amoral character.

Rita: the principle of both natural and moral order in the universe according to the Proto Indo-Iranians.  The gods themselves were said to uphold Rita and observe it.

Dharma: Literally, "that which upholds or supports, Dharma has a meaning similar to that of Rita, but seems to be a younger word than Rita.  In addition to referring the moral duties of human beings it is also often especially used to refer to their ritualistic duties.