Help with skin care routine? What does skin serum do for your skin?

A serum is going to be much lighter and carry potent anti-aging ingredients (if you are buying at the drug store then chemicals, higher end serums will be using botanical extracts) to deeper layers of your skin. A cream is only absorped so much/deeply and apsortion is the key to the success of your moisturizer. A moisturizer is meant to be emollient, may contain a sunscreen and can be anti-aging but a serum is more powerful and focuses on repairing the skin. It is lighter and designed to penetrate the skin more deeply than a moisturizer can.