What is protein in?

Generally everything, but i suppose your question is more concerned with what has the greatest concentration of protein in. The general rule of thumb is that protein is biologically used to build the bodies of animals. So muscle, hair, claws, etc will all contain a high concentration of protein. As far as your diet is concerned, look for all food that has a high animal content. Meat, fish and dairy products are all high in protein concentration (whey protein is a by product of cheese!).

There is one general exception; Soya protein. But I dont rate it, it's biological value is really low and there are reported side effects that induce oestrogen production in men.