IstariThe Istari - Apparently the Valar made the decision never again to intervene directly in the affairs of Middle Earth in the Third Age as they had in the First when they defeated Morgoth (either that or they had become too lazy).  Instead they sent five servants known as the Istari to aid the free peoples of Middle Earth against the threat of Sauron.  They were on the order of Maiar, but Men called them the Wizards.

 

"Even as the first shadows were felt in Mirkwood there appeared in the west of Middle-earth the Istari, whom Men called the Wizards.  None knew at the time whence they were, save Cirdan of the Havens, and only to Elrond and to Galadriel did he reveal that they came over the Sea.  But afterwards it was said among the Elves that they were messengers sent by the Lords of the West to contest the power of Sauron, if he should rise again, and to move Elves and Men and all living things of good will to valiant deeds.  In the likeness of Men they appeared, old but vigorous, and they changed little with the years, and aged but slowly, though great cares lay upon them, great wisdom they had, and many powers of mind and hand.  Long they journeyed far and wide among Elves and Men, and held converse also with beasts and with birds, and the people of Middle-earth gave to them many names, for their true names they did not reveal.  Chief among them were those whom the Elves called Mithrandir and Curunir, but Men in the North named Gandalf and Sarumon.  Of these Curunir was the eldest and came first, and after him came Mithrandir and Radagast..." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

The other two Wizards are not mentioned by name, but the Unfinished Tales lists them as Alatar and Pallando, and they were known as the Blue Wizards (because their robes were blue).  After arriving in Middle Earth they apparently traveled far into the east never to return, and so played no part in the War of the Ring.