Dark meat has more myoglobin (a protein that delivers oxygen to muscle tissues, very similar to hemoglobin) in it than white meat does. The more myoglobin the tissue contains, the darker it will be.
Think of a turkey. Turkeys don't fly, but they do plenty of running around. Their leg muscles (drumsticks) get a lot of use, but their chest muscles (breast meat) do not. Muscles that do a lot of work and require a lot of endurance need more myoglobin than other types of muscle. That's why drumsticks are dark meat and breast meat is white.
As far as the nutritional differences between the two types of meat, dark meat has more fat. This is again because these muscles require more energy (stored in fat) than white meat muscles do. However, nutritionally speaking dark meat isn't all bad. It contains more Vitamin A, B vitamins, and iron than white meat.
Just wanted to give you some food for thought. Now that I've made everyone including myself hungry, I will be signing off.
