What's the Difference Between a Serum and a Moisturizer?

I was recently asked to explain the difference between a moisturizer and a serum.  It's actually a great question for someone who is just starting a skincare routine.  Here's my answer:

Moisturizers or creams tend to reside on the skin's surface and serve as one part moisturizer and one part barrier.  They help protect and restore the skin, but tend to be heavy and thick and therefore, they don't have deep-penetrating capabilities.

Serums are far more powerful and, when used correctly, infuse your skin with hydration from deep below the top layer of the skin.  The consistency of a serum is much thiner than a moisturizer- allowing it to go below the skin's surface for deep hydration.

Serums are powerful topical treatments with intense active ingredients- look for hyaluronic acid/sodium hyaluronate- this is the key ingredient to infusing your skin with deep hydration to get the dewy, glowing skin you seek.  Many serums also have powerful vitamin extracts that promote skin turnover and renewal.

Moisturizers are important in prevention and protection- some also have wrinkle-deminishing ingredients, but serums are the heavy-lifters when it comes to age reversal.

You should apply serums to clean, dry skin and allow the serum to absorb before applying a moisturizer.

My serum pick: 
Kate Somerville's Quench Hydrating Face Serum- this product is great if your are just starting to add a serum to your skincare routine.  It's powerful enough to notice a change in your skin's quality and perfect for proactively combating anti-aging.