When someone introduces himself as "Doctor Jones", does that usually imply he is a PhD, or does it imply he is a medical doctor? Obviously, if you're in the emergency room, a "doctor" can be assumed to be of the medical variety; but what about a more general environment?
In the United States, many people with a PhD will have the title Dr. So I think the rule is you have to ask.
In addition to Medical Doctors (MDs) and Doctors of Philosophy (PhDs), there are also:
In the United States, most lawyers are JDs; they are typically not addressed as "Doctor". Similarly, it is unusual for a teacher with an EdD to be referred to as "Doctor".
In the United States, it can be much more difficult to get into veterinary school than into medical school. On the other hand, most MDs spend more time in apprenticeships after obtaining their MDs than veterinarians do after obtaining their DVMs.
As stated in prior answers, the best advice is to ask. Often people with a Ph.D or a high level of medical education will be upset if you don't use the title correctly.
It should be noted that in common usage in the U.S., however, the term "doctor" is most often associated with medical then scientific/mathematical professions and finally psychology.
Medical Professions (Most Common)
This can include general practitioners (no specialization -- your family doctor), but also specialist doctors such as pediatricians (children's doctors), or those who specialize in specific parts of the body (e.g cardiologist -- heart doctor).
The term "doctor" can also be applied to non-medical professions:
But these non-medical professions are still less common than medical ones and many will assume that if you say "Dr. So-and-So", you mean someone who has a medical degree of some sort. Many academic (and some scientific) professions opt for the title of "Professor", at least if they are teachers.