Gandalf the Grey:

A Maia, sent to Middle-Earth around Third Age 1000, to aid in the fight against Sauron, one of the five Istari, or Wizards, and the Keeper of the Third Ring, Narya, the Ring of Fire, he went by many names:
"The Grey Pilgrim?" said Frodo. "Had he a name?"
"Mithrandir we called him in elf-fashion," said Faramir, "and he was content. 'Many are my names in many countries,' he said. 'Mithrandir, among the Elves, Tharkûn to the Dwarves; Olórin I was in my youth in the West that is forgotten, in the south Incánus, in the North Gandalf; to the East I go not.'" (LOTR.696-697)

Gandalf discovered that it was Sauron growing again in Mirkwood in the year 2850 of the Third Age. Before then, it was feared and believed by the Wise that it was a Ringwraith.

It was due to Gandalf, that the Quest of Erebor, which ended Smaug, was successful, and as a minor adventure on the way (not intended by Gandalf) Bilbo found the magic Ring, later discovered (again by Gandalf) to be the One Ring. Gandalf acted as the leader of the Company of the Ring from Rivendell, through Moria (after the failed attempt on the Redhorn Pass). It was in Moria that Gandalf fell, fighting a Balrog. He was victorious though he died.