Retinol is a derivative of vitamin A. Naturally vitamin A is in carrots, tomatoes and red vegetables. In skin care products, retinol and other derivatives know as retinoic acid, retinaldehyde, and retinal are synthetic compounds developed for their amazing effects on the skin. As a class, they are known as Retinoids and they work by increasing cell turnover, decreasing sebum (oil content of the skin), and decreasing bacterial content. These effects help improve acne, however retinoids have also been proven to increase collagen production in the skin which also visibly reduces the appearance of fine lines and wrinkles.
Retinoids, however, can be very difficult to use. Over-use of the product can lead to redness, irritation and scaling of the skin. Many people say they are "allergic" to retinoids because they have used them either in too high a quantity or too often and have gotten a red, irritated reaction. These products need to be used in small amounts very infrequently with a slow upward titration. I often recommend a pea size amount mixed in with your night moisturizer once a week and slowly increasing the frequency if there is no redness or peeling.
They are wonderful products and provide amazing anti-aging benefits if used correctly!