A facial serum provides added moisture. They differ from moisturisers, day creams and night creams in that they provide added nutrients for the deeper layers of your skin. These nutrients will provide different functions whether you are using a facial serum for anti-wrinkle purposes or to brighten your skin.
Facial serums tend to have smaller molecules than a standard moisturiser allowing them penetrate your skin deeper. Serum should be applied after cleansing and toner and before make up and your normal moisturiser. If you skin is doing fine with your normal moisturiser then you probably will not need a serum as well.