A serum is a concentrated treatment that you apply before your moisturizer. It penetrates the deeper layers of your skin where your moisturizer can't reach. Your moisturizer addresses the top layer of your skin. There a all kinds of serums that address different issues: Hydrating, firming, pigmentation, brightening,illuminating, pore minimizing, etc. I like the Estee Lauder ones and the MAC ones also Caudelie has a fantastic one for pigmentation.