I think there are some variations between US and UK too.
In the past (in the UK) we frequently talk about tramps, but I think that word has lost favour and "homeless person" has come into use. Unlike Packard, I think that "Drifter" seems to denote something more romantic and less poverty-stricken, like a free spirit who maybe has some cash to support a lifestyle on the move.
Vagabond sounds quite old-fashioned. Many of these words seem to denigrate the person, or at least carry negative connotations, especially "bum" which someone else mentioned. I'd say bum and hobo were more American than UK in origin and use.