A moisturizer has larger molecules than a serum.
This means that a moisturizer will not penetrate the skin
as far as a serum will.
While serums deposit nutrients, moisturizers are meant to
deposit moisture and hydration your skin.
Of course skin care companies would like to make you believe
that you need both, but of course that is not true.
If you have a problem with your skin that you'd like to improve,
such as dark spots or early aging signs, a serum can help a lot.
Also, if your skin is very oily, a serum can give you enough hydration.
But, if your skin is happy with your regular moisturizer alone, there is no need.