Antioxidants are molecules that protect cells from destructive external or internal elements. Common antioxidants include vitamins A, C and E and beta carotene.
Antioxidants work by combating free radicals. Free radicals are unstable molecules that are produced by our bodies when we use oxygen. They have the ability to start chain reactions that ultimately damage our cells' DNA and cell membranes (the lining around the cells).
External forces that cause free radicals include air pollution, sunlight, alcohol and cigarette smoke. Intrinsic forces include our normal metabolic process, as well as stress hormones which also result in free radical generation. Antioxidants work by donating an electron and "mopping up" these free radicals so that they can not damage the skin.