I've heard many discussions concerning who Tom Bombadil is, and what he represents. To many, he is a Maiar like Gandalf and some believe him to be Valar or even Eru himself. In the book "The Letters of J.R.R.Tolkien," Tolkien explains Bombadil's role, to a degree.

"Tom Bombadill is not an important person - to a narrative. I suppose he has some importance as a 'comment'. I mean, I do not really write like that: he is just an invention (who first appeared in the Oxford Magazine about 1933), and he represents something that I feel important, though I would not be prepared to analyze the feeling precisely. I would not, however, have left him in, if he did not have some kind of function. I might put it this way. The story is cast in terms of a good side, and a bad side, beauty against ruthless ugliness, tyranny against kingship, moderated freedom with consent against compulsion that has long lost any object save mere power, and so on; but both side in some degree, conservative or destructive, want a measure of control. but if you have, as it were taken 'a vow of poverty', renounced control, and take your delight in things for themselves without reference to yourself, watching, observing, and to some extent knowing, then the question of the rights and wrongs of power and control might become utterly meaningless to you, and the means of power quite valueless. It is a natural pacifist view, which always arises in the mind when there is a war. But the view of Rivendell seems to be that it is an exellent thing to have represented, but that there are in fact things with which it cannot cope; and upon which its existance nonetheless dependes. Ultimately only the victory of the West will alow Bombadill to continue, or even to survive. Nothing would be left for him in the world of Sauron"

This was in answer to a reviewer of The Lord of the Rings who implied Bombadil was Eru himself, or the God of Middle Earth, because when Frodo asks Goldberry who he is, she replies "He is."

"He is MASTER in a peculiar way: he has no fear, and no desire of possession or domination at all. He merely knows and understands about such things as concern him in his natural little realm. He hardly even judges, and as far as can be seen makes no effort to reform or remove even the Willow."

Basically, I think he means Tom Bombadil's character is really just one example of the "good" side in the war of the ring. Tom's carelessness, singing and joyfulness in his happy little realm would be lost if Sauron had his way and found the One Ring. The ring has no effect on him it seems, not because he is god-like and powerful, but that he simply doesn't care at all about items or most anything that happens outside his land. Like Gandalf says in the Council of Elrond, Bombadil doesn't have power over the Ring, it has no power over him, he doesn't wish for any power or to have cares of any sort beyond his realm and the Ring does not tempt or concern him at all. Tolkien also mentioned that Bombadil's other function was to give the Hobbits an 'adventure' along the way.