The sun’s rays, which are called ultraviolet A and ultraviolet B rays (UVA and UVB rays) damage your skin. This leads to early wrinkles, skin cancer and other skin problems. Being in the sun often over time, even if you don’t burn, can lead to skin cancer. A tan is the body’s desperate attempt to protect itself from the sun’s harmful rays.
For more information: http://www.skincancer.org/Sunscreen/