A mole is a skin growth made up of cells (melanocytes or nevus cells) that produce color (pigment). Moles can appear anywhere on the skin, alone or in groups.
Most moles appear during the first 20 years of a person's life. Some may not appear until later in life. Although most moles are harmless, some can become cancerous. Signs of skin cancer include a change in the skin, such as a growth, an irritation or sore that does not heal, or a change in a wart or mole.
WebMD Medical Reference from Healthwise