Hobos vs. Tramps vs. Bums: What is the difference?

​I just learned this today, and the answer apparently is fairly simple.

Hobo: A hobo is a homefree individual that lives a life of freedom, traveling the country and working when necessary.

Tramp: A tramp has the same restless spirit as the hobo, and is ready to move on to the next town on the slightest whim... but unlike the hobo, tramps don't work to support their lifestyle. Fiercely territorial, tramps have been known to mark their passing by applying a stamp to the lower backs of the friendlier female inhabitants of the villages they visit.

Bum: A bum has no home. Doesn't work OR travel... Mostly I think they just like to drink and piss on the sidewalks of San Francisco.