A suntan is a darkening of the skin in reaction to exposure to the ultraviolet (UV) rays in sunlight. The skin pigment melanin becomes darker as UV rays damage the skin.
Although skin gets darker to protect its deepest layers against harm from UV rays, a tan is evidence of some degree of sun damage. This damage is cumulative, which means the more time a person spends in the sun over many years, the more long-term damage is done to the skin.
Tanning booths also can cause skin changes and damage.
WebMD Medical Reference from Healthwise