Whey Protein Concentrate
Whey protein powder is manufactured by forcing whey past a very fine filter. The filter will allow the non protein materials and water to pass through it, but the protein molecules are larger and left behind in the filter and used to form the Protein Powder called whey Concentrate.
Whey Protein Isolate
Whey Isolate is acquired in the same way but undergoes another process ( cross-flow micro-filtration ) giving a more pure and lactose free protein powder we call whey Isolate.
Whey isolate is generally more expensive. The body digests whey isolate and whey concentrate at different speeds which is why some bodybuilders chose a blend of whey concentrate and whey isolate together.
Individuals who are lactose intolerant should chose whey isolate and some companies actually guarantee that their whey isolate protein powder is 100% lactose free. generally whey isolate will contain 4% or more lactose.