Component of urine, although synthetic versions are used in cosmetics. In small amounts, urea has beneficial water-binding and mild exfoliating properties for skin. In larger concentrations it can cause sensitivity, though large amounts can also exfoliate skin.
Urea can enhance the absorption of other cosmetic ingredients.
References for this information:
The Art and Science of Moisturizers, Springer Berlin Heidelberg, 2012, pages 481-492
International Journal of Toxicology, 2005, (S3) pages 1-56
American Journal of Clinical Dermatology, March 2002, pages 217-222
Skin Pharmacology and Applied Skin Physiology, January-February 2002, pages 44-54