Until the 1970's, -all- graduating medical students did an "internship." They were very similar to the current transitional year residency or osteopathic rotating internship year. During that year, they decided on their specialty and then went into residency afterwards. Internships were eliminated in 1973, I believe, and became the first year of residency.
Internship is the first year of residency, but the term "intern" has generally fallen out of style. Though the old-school docs who actually did an internship might still call PG1's "interns," I would say that most are simply "residents," just like PG2's, 3's, etc.
Despite the Bell laws, I do not believe that first year residents in New York work any less than most others across the country.
If you want to know the average number of work hours/week for all specialties, go to the FREIDA search engine (linked from the AMA's web site). You can find all that information there.
Yes, IM residents rotate through different subspecialties.