What in the world is the difference, if any, between a Ground Hog and a Woodchuck?

I'm from Boston, and in the middle of the city, crossing the steet (jay walking against the lights) last week I saw an animal I've never seen before. It was short, fat, wide, and was brown like a beaver and it had a short furry brown tail. Quite cute, and it look lost as can be. What was it I saw?
Update: Yeah, thanks. It was a Woodchuck/Ground Hog I saw crossing the street heading toward the Charles River Esplanade. WOW. But what was it doing here???
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