We all have our answers and theories, but for those of you who are unfamiliar with the character, here he is:

He described himself as Eldest, existing before anything else in Middle-Earth, and has strange powers to master those who threaten him with song and verse. Nobody knows what he is. What's more, he could not affect the ring, and the ring could not affect him! However, it is said that if Sauron got the ring, he would, even if it took him centuries to do so, and likely would have, defeat Tom Bombadil.

However, we know what he represents. Tolkien described Tom as being representitive of the vanishing countryside of Oxford and Berkshire, embodied and existing in Time, and in some continuous cycle with Goldberry, who apparently represents the changing seasons, and he is also representative as someone who has taken "a vow of poverty" . and just studies things and enjoys them for what they are, not as how he can control them.

While that means that he's not a Man, a Hobbit, Elf, Dwarf, or God, it automatically opens up a whole new wave of possibilities, being one of the Ainur, a Maia or a Vala, or that he is "Father Time", or the incarnate of the Music of the Ainur, or he is exactly what one immediately thinks of from the first part of the description: An Avatar of Earth itself, perhaps the landscape, the forest, the hills, a "Father Nature", or even an Avatar of Matter itself!

What do you think? To be honest, I don't know...and neither did Tolkien. He said that Tom was a "mystery to the author himself". I don't think we're supposed to be entirely sure, and just enjoy the character and think about the commentary he brings.

I always viewed him as sort of the embodiment of the forest, which would mean he would live as long as the forest was still around.

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To be frank, I suspect something like that myself: An embodiment of some natural feature of Middle-Earth. It explains everything, even the hint that he was there before the Ainur explored Middle-Earth.