A people doctor was a slang term for a doctor who cared for sentient lifeforms, particularly Humanoids, rather than animals (such as dogs), as a veterinarian would.
In 2152, Jonathan Archer was surprised by the level of Dr. Phlox's medical training in Interspecies Veterinary Medicine (six degrees), as he had believed Phlox to be a people doctor. (ENT: "A Night in Sickbay")