Dual MD/PhD programs are sometimes known as physician scientist training or medical scientist training programs (MSTP) (especially if the program is funded by the National Institutes of Health).
These programs combine MD and PhD training into one integrated program. So students attend medical school to obtain the medical degree and a graduate program to get PhD training.
The resulting combination of MD and PhD degrees trains the physician scientist to both practice medicine and acquire the research skills of a PhD. Hence these physician scientists or medical scientists are able better understand and do research into the causes of diseases in their patients and work on new therapies to help their patients.
The format for the MD and PhD training does vary from program to program but typically the first two years are spent in medical school, the next three years the students work on their PhD and then return to medical school for the final two years.
The schematic below shows the format for this type of MD PhD program.