The name for the entire vitamin A molecule. Retinol has value for skin on several fronts: it is a cell-communicating ingredient and antioxidant. Skin cells have a receptor site that is very accepting of retinoic acid, which is a component of retinol. This relationship between retinoic acid and skin cells allows a type of communication in which the cell is told to function normally (that is, not like a damaged or older cell), and it can, to some extent, conform to that request. That's one of the reasons retinol is an exciting anti-aging ingredient.
Retinol cannot communicate with a cell until it has been broken down into retinoic acid. Some of the controversies regarding using retinol in skin-care products have been its stability in skin or in a skin-care product, whether it can be converted into retinoic acid after it is absorbed into the skin, and how much retinol is needed so that as it is changed to retinoic acid there is still enough that can get to the cell. However, within the last few years, more stable forms of retinol have become available, along with lots of impressive research regarding their efficacy.
Retinol helps skin cells create better, healthier skin cells, provides antioxidant support, and increases the amount of substances that enhance skin’s structural elements. Packaging is still a key issue, so any container that lets in air (like jar packaging) or sunlight (clear containers) just won't cut it, something that applies to most state-of-the-art skin-care ingredients. Lots of retinol products come in unacceptable packaging; these should be avoidede because the retinol will most likely be (or will become) ineffective.
Many consumers are concerned about the percentage of retinol in anti-aging products such as serums or moisturizers. Although the percentage can make a difference (especially if it's too low) it is not helpful in understanding how a retinol product will benefit your skin. Far more important is the delivery system, package stability, and what other ingredients the retinol is paired with. Using a product with a range of anti-aging ingredients plus retinol is far more valuable for skin than using a product with only a supposedly high percentage of retinol (or vitamin C, or niacinamide, or green tea, or ubiquione, and on and on). Skin is the largest organ of the body and needs far more than any one ingredient can provide. It doesn't make sense to fixate on the percentage of retinol when so many other elements are also important.
What about claims of retinol being helpful for cellulite? Since retinol is one of the ingredients known to help improve skin structure, it has some value in anti-cellulite products. Of all the ingredients to look for in a cellulite product, this should be at the top of the list. However, most cellulite products contain only teeny amounts of retinol (at best) and are often in packaging that won’t keep this air-sensitive ingredient stable. In skin-care products, it is found in the form of retinol, retinyl palmitate, and retinylaldehyde. In prescription-only skin-care products, it is in the form of retinoic acid (also called tretinoin). Sources for the information above: Journal of Drugs in Dermatology, July 2009, pages 674–677; Critical Reviews in Food Science and Nutrition, April 2009, pages 361–368; Archives of Gerontology and Geriatrics, November-December 2008, pages 356–367;Archives of Dermatology, May 2007, pages 606-612; Cosmetic Dermatology, Supplement, Revisiting Retinol, January 2005, pages 1–20; Dermatologic Surgery, July 2005, pages 799-804; Plastic and Reconstructive Surgery, April 2005, pages 1156-1162; Mechanisms of Ageing Development, July 2004, 465-473; Applied Spectroscopy, September 2003, pages 1,117–1,122; and Journal of Dermatology, November 2001, pages 595-598).