They're gypsies who travel around the country doing handyman work. Unfortunately, they're also con men. They say they'll do something and then either do a crummy job or just skip out altogether. Their MO is stuff like going up to a target house -- usually the elderly -- and saying they were just driving by and noticed that they have a bad roof/chimney/whatever and "since they happen to have some materials left over from a job they just finished," they'll do the job cheap. Oh, and could you pay up front please.
The police look on them with extreme disfavor around here. They blow in en masse about once every five years or so, scam whoever they can and disappear without a trace...
Yes, I have heard that the Travelers are expecially expert in roofing and repaving jobs, that are never finished or are poorly done, if at all.