Answer:
Most skincare companies compose their lines of the following basic products; cleansers, toners, astringents, serums, moisturizers, and sunscreens.
A "beauty serum" is a liquid concentrate of active ingredients meant to target something specific in the skin. For instance, a serum might consist of peptides and vitamins to target anti-aging.
After cleansing, one would apply a small amount of a serum (based on their skin type) and massage it into the skin. It generally absorbs quickly and can sometimes be layered with other serums or you might apply a moisturizer or sunscreen over the top depending on what kind of results you are trying to achieve.