Clarifying shampoo is a deep cleanser that uses heavier surfactants than regular shampoos to eliminate grease, product residue, and impurities from your scalp. It's a heavy-duty cleanser that you can use to reset your hair every once in a while.  
This shampoo is a favorite. It is great to bring when traveling and because it is PH-balanced you don't have to worry about overusing it. It must be left on your hair for 3-5 mins for the clarifying process to work best.