(In an email answer to the controversial question, "Is Tom Bombadil a Maiar?" Date may have been around 1999.)
Tom Bombadil, or Iarwain Ben-Adar, was indeed one
of the Maiar.
He was chosen by Yavanna in the Spring of Arda to be caretaker of the wild wood which covered most Middle-earth at that time. He was also the first of the Maiar to enter Ea.
Within the boundaries of the Old Forest which he called home, he was amongst the most powerful of the Maiar spirits; indeed his power exceeded that of Sauron (though Sauron would be considered his equal when wearing the One Ring). Only Eonwe and Arien held greater power than Tom, and it was this great strength which allowed him to resist the One Ring.
Remember that Sauron did not put all of his essence into the Ring, only part of it, and that part not sufficient to overpower the chief Maiar of Yavanna's people.
This is an interesting subject, of which little is known beyond what I have outlined above.