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.
Because it's thinner, more tender, and more exposed to the heat of the oven, the white breast meat usually finishes cooking first. The more compact and sheltered dark meat takes longer.
One of the tried and true methods of cooking a turkey is to start it upside down so the breast is sheltered and the wings, thighs, and drumsticks are exposed, and then turning it right-side up partway through cooking. Other methods involve shielding the breast meat with aluminum foil or barding it with strips of bacon to keep it moist.
What's your method for cooking a turkey evenly?