Who is Gandalf from Unfinished Tales of Numenor and Middle-earth and what is their importance?

Known as the Grey Wanderer, Gandalf is one of the Istari sent to Middle Earth by the Eldar. He plays a key role in the finding of the One Ring and in the overthrowing of Sauron. During the quest to destroy the Ring of Power, Gandalf sacrifices himself to save the Fellowship. He dies but is brought back as an even greater power, clothed all in white.