Retinol is a pure derivative of the active molecular form of Vitamin A. It has been shown to increase cellular turnover and stimulate collagen production, which can lead to healthier, smoother, and younger-looking skin. It is also useful for the treatment of acne, as the stimulation of cell regeneration helps to clear clogged pores.
It is available in low concentrations in many over-the-counter products. Higher doses are available for more severe skin issues, but they're available by prescription only, since the overuse of Vitamin A can lead to a number of health problems including birth defects. Regardless of whether you go with a mild or stronger form of retinol, wearing sunscreen is imperative.