Moles, or naevi, are brown or tan spots on the skin that result in the clustering of melanocytes (pigment cells of the skin).
Melanocytes (cells that produce pigment) are present in the lowest layer of the epidermis- the outer barrier of the skin. With certain forms of stimulation; such as the ultraviolet irradiation (i.e. usually from sunlight), the density of melanocytes in normal skin may increase.
Melanocytic naevi represent normal, harmless proliferations of melanocytes that are in contact with each other, forming small collections of cells known as nests. They are common lesions found on the skin of almost all individuals. Some patients have a few naevi, while others have hundreds.