As others have noted, and as you may suspect, they ARE different than medical school interviews.
I can recall only one (for surgery) at which I was pimped (Christiana...where I was asked by the former Chair what Ringer's Lactate contained) and only one place that questioned me about being a little older than the other candidates.
The goal is to really see whether or not you would fit in. Each program tends to have a "mind-set", a feel if you will. Residents who don't fit that mold tend not to be happy. My program, for example, tended toward down to earth types. Those who couldn't relax or seemed stiff were felt not to be good fits (too bad I guess if you were just nervous that day). Other places wouldn't want jokesters or are more interested in training researchers rather than clinicians.
They really are much more relaxing but do beware, all "social" events, tours, etc. can be fodder for evaluating you...you are always being interviewed, no matter how informally.