His nature is quite complicated. People far wiser than me have not been able to decipher what he is. If you do a search on Google, you'll find a buttload of essays. Most of them are quite interesting to read.
Tolkien said:
"And even in a mythical Age there must be some enigmas, as there always are. Tom Bombadil is one (intentionally)."
That is quite strange for him, as he on all other matters has described the nature to the greatest of detail.
I don't know what he is, but it is my guess, that he is some sort of spirit of nature, a part of the world from the time it was created.
In contrast, if Tom is a nature spirit, then no vow of poverty has been taken, and there is no natural pacifist view. According to the nature spirit thesis, as Veryln Flieger puts it in Splintered Light, published in 1983: "Tom Bombadil, on whom the Ring has no effect, is a natural force, a kind of earth spirit, and so the power over the will which the Ring exerts simply has no meaning for him" (p. 128, note). As a natural force, Tom has the same status as a falling rock or the wind or the rain - he is blind activity with no direction or purpose. As such he is not a moral agent, and cannot therefore make moral decisions. The moral dimension is thus completely absent. Tom is immune to the influence of the ring not because of his high moral character, but because he is not capable of having a moral character at all.
This quote was taken from this essay, wich is a mouthfull, but worth the read (Atleast the parts I have read are interesting and well-founded). Also check this one.
I'm not really sure what he represents outside of the Tolkien mythology, but it would be an interesting study.