Whey is the liquid part of milk that is separated from the curd when making cheese (remember little miss muffit).
It is naturally low in fat, lactose, cholesterol, and carbohydrate - it fits into the low fat diet and the low carb diets. It is also not going to be as harsh on those who can't tolerate lactose as milk protein would be.
It is a
complete protein - that means it has ALL of the
essential amino acids and when it is digested, they make it into the blood and on to the muscles. If you take amino acids directly instead as protein suppliments they are digested and passed out of the body. I learned this when I was a guinea pig for Teva Neurosciences and took oral copaxone instead of the injectable (the medicine is simply 4 amino acids in a subcutaneously injectable form) and most of us on the oral form got no benefit from it because we digested and excreted it all.
It is a very soluble protein so it mixes easily. It is easily digested and gets quicly to the muscle tissues. It is believed to make it to the muscles quicker than any other protein form.
Whey increases glutathirene - an antioxidant which boosts the immune system. Weight training is believed to reduce this antioxidant.
Whey slows absorption of glucose into the bloodstream. The more stable glucose release lowers insulin levels and helps reduce fat storage. This is good for those on a glycemic controlled diet like any of the ketogenic plans (atkins, south beach, mine). For those of us who spike, the whey is quickly pulled into the muscles by the spike just like any other protein and when the insulin levels go back down the whey doesn't cause them to go up again at an unplanned, undesirable time.
There is one drawback to whey that I can find, it doesn't have the creatine like meat protein does. So, I strongly reccomend occasionally supplimenting your whey with the occasional rib eye. Or if you are serious enough about your diet do what I do and suffer through the round steak or chuck steak with the fat cut off.