Whey protein concentrate is widely considered to be the most efficient and economical form of protein for the human body to digest and use. Whey protein concentrate is micro-filtered and not heat treated to produce ultra pure protein. That means a bit more carbs & fat and about 15% less protein per serving.
If you are lactose intolerant or currently watching calories carefully whey isolate will be the better product for you. Isolate and concentrate is basically the same protein, but the isolate is filtrated more which means zero fat, zero sugar & about 15% more protein per serving.