Egg protein, casein, and whey protein; there's even beef protein out there. There's all different types of protein and what's really popular today is to take a lot of these different proteins and put them into a blended formula, there by giving you a little bit of each of the proteins. For instance casein, which is a slower acting protein, is now mixed with egg and whey and soy on.
There reason why you may find some of these proteins useful in blends has a lot to do with two things; one has to do with the digestibility of these blends, and two is the texture of the protein. Some people don't like to drink protein powder and it's not because it maybe doesn't agree with their stomach, but because they think it tastes chalky and disgusting. They'll switch to a blended formula or a different type of protein and like the way it does down, and the way it responds on their pallet.
Protein blends are going to give you a different type of digestibility with your protein because whey and egg are very, very fast acting where as casein is slower. It's going to slowly distribute the protein into your bloodstream at a slower rate and it's really really good to keep your body anabolic to help you to build muscle in those moments when you may not be able to get protein into your system down the road again quickly.
But then, it's also that texture factor. Some protein powders are created at a much higher quality and that can even affect the texture and the taste of the protein itself. Sometimes it can change the digestibility of the protein. So, you want to try out and see what works best for you.