The concept of a skin serum is not a medical one. This concept was created by cosmetic companies and the esthetic industry as a way to market their products. The beauty industry considers a serum to contain smaller molecules or nutrients that will penetrate into the skin easier than a plain moisturizer. The problem with this idea is that the epidermal-dermal junction of the skin possesses sub-microscopic barriers that do not allow these so-called nutrients to penetrate into the skin dermis, in most cases.
A percentage of dermatologists feel that components in the serums may temporarily reduce wrinkles and signs of aging. Popular "skin nutrients" include vitamin C and other anti-oxidants, growth factors, retinoids, glycolic acid, and many more. I find that some of my patients enjoy using some of these products and do feel that their skin feels better, looks brighter, tighter, less sun damaged, and less blotchy with daily use.
However, the best moisturizers are the ones that make your skin feel less dry. Most dermatologists favor Cetaphil, Cerave, Eucerin, or Neutrogena moisturizers, which are safe and cost-effective. If you have acne-prone skin, look for an oil-free moisturizer. As always, the best moisturizers that can help prevent aging are those that contain sunscreen.