Istari is Quenya and means "The Wise", "Wizards", sg. Istar;Also called:Sindarin: Ithryn (sg. Ithron)
Maiar that were members of the Heren Istarion, "Order of Wizards". They came to Middle-earth around the year 1000 TA, sent by the Valar to agitate among the Elves and Men against Sauron and resist him in secret, should he rise in power again.
Although they were Maiar, they were clad in real bodies and suffered in the same ways Men did, except they didn't die of old age. They all had the shape of old men that aged very slowly.
The Istari were five and were, as a token of their order, clad in different colors. The first had a white cloak and was in Valinor called Curumo, though the Men of northwestern Middle-earth called him Saruman and the Elves called him Curunir; he was the leader of the order. The second and third were the Ithryn Luin, the Blue Wizards dressed in sea-blue. Their names in Valinor were Alatar and Pallando, but they gained no names in the northwest of Middle-earth since they didn't stay there. The fourth was clad in brown and was called Aiwendil, by Men Radagast. The last one was Olorin, called Mithrandir by the Elves and Gandalf by Men, and his cloak was grey.
In the end only one stayed faithful to their mission, because Saruman turned to evil, and the Blue Wizards disappeared into the east, and Radagast "became enamoured with the wild creatures of Middle-earth" and neglected his mission. But Gandalf, after having accomplished his task and becoming the White at the end of the Third Age, left Middle-earth and returned to Valinor. What finally happened to the other Istari is not known.
Unfinished Tales: The Istari
The Tolkien Encyclopedia
The Encyclopedia of Middle-earth
Hypertextual System by FMI Publishing