To you, the difference is a matter of taste, but to the bird, the difference could help them fly the coop.
Next time you tuck into a turkey dinner, you may wonder how one animal can taste so different depending on the body part. The reason for this peculiarity is that turkey and other birds need two different types of muscle: endurance muscle and strength muscle.
Dark meat is the endurance or "slow twitch" muscle. Because turkeys stand on their feet for long periods of time and fly only rarely, dark meat is located in the legs and thighs.
What gives it the dark color is the presence of myoglobin, a pigmented protein similar to the hemoglobin that transports oxygen in your blood. Because oxygen is more attracted to myoglobin than hemoglobin, slow twitch cells can "steal" oxygen circulating in the bloodstream and use it to burn energy. This energy primarily comes from fatty acids (slow-burning sources of energy) which is why dark meat is so moist and tender — it's full of fat!
Light meat, on the other hand, is the strength or "fast twitch" muscle. You'll find it in the breast and wings of a turkey because they only fly for short distances.
Fast twitch muscle doesn't need myoglobin to acquire oxygen because a short sprint doesn't use any more oxygen than is already stored in the tissue. This is why sprinters like Usain Bolt aren't out of breath when they finish the 100 m dash. They power their muscles with hardly any oxygen at all because the aerobic (oxygen-burning) method of generating energy is an efficient but slow process. Because these muscle cells need energy FAST they get their energy from blood sugar (not fat) so light meat is dry and flaky.
So why don't other animals we eat have light and dark muscles? Actually, they do. The two muscle types might not be as clearly contrasted as in a turkey because they are either mixed-in together or because the animal relies primarily on one type of muscle. Take beef for example — ever seen a cow sprint?
Cows are almost entirely made-up of dark, "slow-twitch" muscle because they spend all day standing and don't require muscles for sprinting. Like the slow-twitch muscles in turkeys, beef is rich in fat so that they can keep burning energy all day long.
Learn more about muscle types.