Many reactions result from sunlight shining on the skin. Melanocytes are stimulated, which in turn produces melanin, the pigment that gives your skin its individual color.
Sunlight causes freckles, the color in some moles, and potentially mild to severe sunburn.
Finally, DNA can be altered and may form malignancies later on. Melanin absorbs UV light and is produced by your body in an attempt to protect itself from radiation. A tan is the body's way of responding to sun damage. When you see people walking around with a nice tan, they are literally displaying the body's magnificent response to danger. If you have a tan, you have sun damage.