Whey protein powder is derived from just that, WHEY. When cheese is made there are curds and whey (just like the nursury rhyme remember "Little Miss Muffet sat on her tuffet, eatting her curds and whey". The curds they use to make cheese, the whey, up until a few years ago, they threw out as a useless by-product, until one day some brainiac happened upon realized that whey was actually a very high quality protein.