To distinguish where the meat came from within the turkey. White meat is typically derived from the breasts while any meat derived from the thighs, drumsticks or "back" are considered "dark" meat.
White meat is considered less fatty while dark meat tends to be more fatty.
From a [url=http://www.thekitchn.com/food-science-the-difference-be-69729]cooking source[/url]:
When we talk about the white meat on a turkey or chicken, we're talking about the breast meat. It's white because the fuel for these muscle fibers comes primarily from a carbohydrate called glycogen and doesn't require as much oxygen from the blood. Glycogen is useful for short bursts of activity, like the short distance a turkey flies when startled.
Dark meat is found in the wings, thighs, and drumsticks, and it's actually a different kind of muscle than the white meat. These muscles get their fuel primarily from fat, which provides a more sustainable energy (for the prolonged activity of standing, walking, and running) than the glycogen. Dark meat is made dark by two proteins involved in the process of converting the fat into energy for the muscles.
Dark meat has a stronger and more game-like flavor as compared to white meat. This is due to the activity of those muscles and the various chemicals, proteins, and fats that activity builds in the muscle tissue.