By definition a hobo is someone who travels around the country, train hopping, looking for work. If he's not working, then he's just a traveler, or a tramp.
A hobo travels looking for work.
A
tramp travels
not looking for work.
A
bum stays in one spot (
more or less) not looking for
work.
In cities like
Chicago, bums are fairly easy to find, although the word 'hobo' is often used for all three (hobo, tramp, and bum).