No one knows who Tom is.  .Is he one of the elven folk, Maia, Valar or Iluvuta himself? Well, he is not an elf because in the Fellowship of the Rings, it said: "when the elves passed,  Tom was here already." 

MAIA: There is no direct evidence for this- he can't be a Valar so he must be a Maia-but he might be a valar,and there is at least one other posibility for him to be a Maia.  The ring had NO EFECT ON HIM, for it said, "Then Tom put the ring around his little finger and held it up to the candalelight... There was no sight of Tom disappearing!"

More information can be gathered from the website:  The Encyclopedia of Arda.  Here is the link:  http://www.glyphweb.com/Arda/default.asp

What are your feelings about Tom Bombadil?  Please comment.