What is the difference between whey protein isolate and concentrate?

Isolate goes through more processing and removes most of the lactose and fat, leaving basically just protein (like 90%+), while concentrate has simpler processing and keeps more lactose and fat (around 70-80% protein). In practice, isolate is better if you're lactose intolerant or want something more "pure," but concentrate is cheaper and the difference isn't really that big if your goal is just to build muscle. Which one you choose kind of depends on your budget and whether lactose affects you or not.