There are already some nice topics about him -.-
Nobody knows who he really was but what is right for sure is that he was some old guy living in the Old Forest (near Bree) who spoke to trees and wasnt affected by the Ring (didnt become invisible when he put it on - heeey like sauron also didnt)... some think he's just some crazy guy, some think he's a Maia or Ilúvatar himself, well, who knows. In fact he was a not-very-liked doll of Tolkien's son (who tried to put it down the drain through the toilet)