Freckles are little brown spots on the skin that are caused due to sun. The melanocytes (pigment-producing cells) produces melanin pigment and feed it to keratinocytes, the normal skin cells that form the outer barrier of the skin, which protects us from the harmful UV rays. Therefore, when the melanocytes become damaged by the sun, the freckles start to occur, which are simply abnormal collection of melanin pigment in the skin.
It is more common amongst people with fair complexion. The areas where they are most commonly observed are the face and the other exposed parts of body like neck, shoulders, chest etc.
Causes of freckles include genetics, diseases (such as xeroderma pigmentosum, a rare disease that causes an increased sensitivity to ultraviolet light, such as the sun), and exposure to the sun.