A serum It's a softer medium where the molecules are much smaller, so it goes faster into your skin, making it an ideal medium for deeper treatment and in preparation for the rest of your skincare routine.
Normally a Serum would be very liquid in a gel or oily form. Most of the deeper treatments would be either a serum or enhanced by the use of Serum.
Why would you combine it with other products? Maybe using only a Serum could work in some cases, but in many a serum would be mostly to help your skin absorb the rest of the treatment. A good serum is basically like a straw that allows your skin to "drink" the moisturizer and treatments in a much more efficient way. In a few words, a Serum is an enhancer for your skincare routine.
To choose the right serum for you, focus on your biggest skincare concern, fixing it would be its main function. You can always complement it with your moisturizer and other skincare products.
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