Well, nobody knows who he is for sure.
There are lots and lots of speculations. Some say he’s Eru himself or one of the Valar, others that he’s Sauron or some other powerful and foul creature. Then of course, he could just be Tom Bombadil.
He really is a mysterious character though. He seems to be on Middle-earth since its creation (x) and the Ring has no powers over him.
But maybe that wasn’t even your question. ^^
To put it simply, Tom Bombadil is an elderly, vivid, cheerful man who lives in the Old Forest with his mysterious elvish-seeming ‘River Daughter’ wife Goldberry. He’s the master of the forest and the Barrow-downs and doesn’t want to have anything to do with what lies outside of this area.
He appears in the chapters ‘The Old Forest’, ‘In the House of Tom Bombadil’ and ‘Fog on the Barrow-downs’ in The Fellowship of the Ring and helps Frodo and the other three hobbits on the beginning of their journey.