I get this question all of the time, It's a great one. What is the difference between a serum and a moisturizer?
A serum, essentially works to repair or fix an issue. Think dehydration, wrinkles, elastosis (sagging or loose skin), or sensitivity and redness. Serums cost a bit more because they cost more because the ingredients are harder to come by or make. The molecule size of serums is smaller and readily absorbs all the way to the dermis to effectively make some good changes. To use a serum, apply it to clean dry skin before your moisturizer. Serums are not enough by themselves, you still need a good moisturizer.
A moisturizer or face cream works on the surface of the skin to keep it balanced and in excellent shape. It will keep your skin from something called Trans Epidermal Water Loss, this is bad. It means skin depleted of water on the cellular level and can lead to premature aging. Sometimes this can happen for runners as well, due to the elements. So slather on a lot of cream, I mean a lot, before the run. Or an oil to create an occlusive barrier, this will help immensely.
Moisturizers keep skin from feeling tight and dry. They will also help with fine lines, dehydration, acne skin types, think about it working on the surface to perfect skin.
Always use a good moisturizer, and when needed, use a serum too. A hydration serum can be used all the time. I love love love hyluronic acid for this purpose.
Remember you don't need creams and serum to be beautiful, you already are. Those things just keep the outside nice.