A hobo is an itinerant worker, a tramp is an itinerant non-worker and a bum is a non-itinerant non-worker.
A hobo was a member of a distinctive sub-culture of homeless, travelling workers in the United States during the late nineteenth and early twentieth centuries. They were particularly associated with the railroads, as they had the reputation for hitching free rides from place to place in the baggage cars of trains.
The population of hobos increased during times of economic trouble, and their numbers increased greatly during the Great Depression. With no work and no prospects at home, many decided to travel and try their luck elsewhere.
Nowadays there are few railroad-riding hobos left, though there are still small numbers of them.
Life as a hobo was a dangerous one. In addition to the problems of being itinerant poor far from home and support, and the hostile attitude of many train crews, the railroads employed their own security staff, often nicknamed bulls. These showed little mercy to hobos they found. If that wasn't enough, riding on a freight train is highly dangerous. One can easily fall under the wheels or get trapped between cars, or freeze to death in bad weather. Hobos tended to band together for protection and formed an informal 'brotherhood'.
Hobos in fiction: