What exactly was Tom Bombadil?

I genuinely don't understand what Tom Bombadil was. Was he some sort of Maiar? Someone created by the Valar? What was he exactly? As far as I can tell, he was really old, older than all the elves, a being from the time Morgoth and Numenor. However, his power was confusing. He seemed to have immense power within the Old Forest and the Barrow-downs while being immune to the effects of the rings of power, including the One Ring. But did his power go beyond that? Mainly, what was he?