The title Doctor (abbreviated Dr.) is used for anyone who holds a doctorate. In the United Kingdom and Australia, physicians are almost exclusively referred to (apart from the manner of address used for everyone with a doctorate) as "doctors." This is done likewise in the United States, but the two terms are commonly used interchangeably.
In the UK and other Commonwealth countries following the UK system, not all "doctors" are Doctors of Medicine. The academic doctorate in Medicine is a higher doctorate which is only earned by those who have made a significant academic contribution to medical science. Note also that surgeons are referred to as "Mr", not "Dr", unless they are also Doctors of Medicine.