Freckles are small brown spots that appear on the skin, often in areas that are exposed to the sun. The majority of the time, freckles are completely harmless. They form due to an overproduction of melanin, which controls pigmentation for skin and hair color. Freckles appear when the skin is stimulated by ultraviolet (UV) radiation. Some people have freckles that fade away during the winter and reappear in the summer.