Melanoma is a cancer that appears to start mostly in the skin. The skin has two main layers: the epidermis and the dermis. Deep in the epidermis are cells called melanocytes. Melanocytes make melanin, which gives color to the skin. When skin is exposed to the sun, the melanocytes make more melanin and cause the skin to tan or darken. Sometimes melanocytes cluster together and form moles, called nevi. Moles are common and are usually not cancerous.
There are three types of skin cancer: basal cell carcinoma starts in the basal cells; squamous cell carcinoma starts in the squamous cells; and melanoma starts in the melanocytes.
Melanoma is less common than basal and squamous cell carcinomas. It can start in other places in the body where melanocytes are found, such as the eyes, the mouth or under the fingernails, although these types of melanoma are rare.