Component of urine, although synthetic versions are used in cosmetics. In small amounts urea has good water-binding and exfoliating properties for skin; in larger concentrations it can cause sensitivity, though large amounts can also exfoliate skin.
References for this information:
Skin Pharmacology and Applied Skin Physiology, January-February 2002, pages 44-54
American Journal of Clinical Dermatology, March 2002, pages 217-222
The Art and Science of Moisturizers, Springer Berlin Heidelberg, 2012, pages 481-492