The Istari ("those who know"), who some call wizards, are emissaries from the Valar, sent to aid Men and Elves in their struggle against Sauron. They appear to be old Men, but that is deceiving. Initially some thought them to be Elves due to their longevity and wisdom. For that reason the one the Eldar call Mithrandir is known as Gandalf ("Wand-elf") by Men in the northwest of Middle-earth.
There are four other Istari besides Mithrandir. Curunír ("skilled one") and Radagast ("tender of beasts"), as well as those reportedly called Alatar and Pallando who travelled into the East. The fate of these latter two is unknown.(1)
Curunír, who is known in Rohan as Saruman ("Man of Skill"), has chiefly been concerned with the Men of Gondor. In TA 2759 Steward Beren, as a sign of friendship and trust, gave into Saruman's care the key to Orthanc, the tall, black tower that stands amid the fortress of Isengard located at the southern tip of Hithaeglir ("Misty Mountains") at the source of the River Isen.
Within Orthanc resided one of the seven palantíri, or seeing stones, that Elendil brought to Middle-earth from Númenor. These crystal globes were used to communicate with each other and to view events from afar. It was feared that Sauron acquired a palantír after the Úlairi (or "Ringwraiths") captured Minas Ithil. It is believed that by using the Orthanc stone Curunír, under the Enemy's influence, turned to evil.
Radagast the Brown makes his home in the western edge of the Greenwood in a place known as Rhosgobel. He is far more interested in the welfare of birds and beasts than in the affairs of Men, Elves, Dwarves, or Halflings.
The Istari began arriving in Middle-earth around the year TA 1000, shortly after evil began stirring in Greenwood the Great. The evil was first attributed to someone called the Necromancer, who was later revealed to be our enemy of old, Sauron. It was around this same time that Halfings began a westward migration from their homes east of the Misty Mountains.
(1) J.R.R. Tolkien, Unfinished Tales
