The following is provided by the National Cancer Institute

Melanoma is the most serious cancer of the skin. It begins in certain cells in the skin called melanocytes. In some parts of the world, especially among Western countries, the number of people who develop melanoma is increasing faster than any other cancer. In the United States, for example, the number of new cases of melanoma has more than doubled in the past 20 years.

Melanocytes are found throughout the lower part of the epidermis. They produce melanin, the pigment that gives skin its natural color. When skin is exposed to the sun, melanocytes produce more pigment, causing the skin to tan, or darken.

Sometimes, clusters of melanocytes and surrounding tissue form benign (noncancerous) growths called moles. (Doctors also call a mole a nevus; the plural is nevi.) Moles are very common. Most people have between 10 and 40 of these flesh-colored, pink, tan, or brown areas on the skin. Moles can be flat or raised. They are usually round or oval and smaller than a pencil eraser. They may be present at birth or may appear later on--usually before age 40. Moles generally grow or change only slightly over a long period of time. They tend to fade away in older people. When moles are surgically removed, they normally do not return.

Melanoma occurs when melanocytes (pigment cells) become malignant. Most pigment cells are in the skin; when melanoma starts in the skin, the disease is called cutaneous melanoma. Melanoma may also occur in the eye and is called ocular melanoma or intraocular melanoma. Rarely, melanoma may arise in the meninges, the digestive tract, lymph nodes, or other areas where melanocytes are found.

Melanoma can occur on any skin surface. In men, it is often found on the trunk (the area from the shoulders to the hips) or the head and neck. In women, melanoma often develops on the lower legs. Melanoma is rare in African Americans and others with dark skin. When it does develop in dark-skinned people, it tends to occur under the fingernails or toenails, or on the palms or soles.

The chance of developing melanoma increases with age, but this disease affects people of all age groups. Melanoma is one of the most common cancers in young adults.

When melanoma spreads, cancer cells are also found in the lymph nodes (also called lymph glands). If the cancer has reached the lymph nodes, it may mean that cancer cells have spread to other parts of the body such as the liver, lungs, or brain. In such cases, the cancer cells in the new tumor are still melanoma cells, and the disease is called metastatic melanoma rather than liver, lung or brain cancer.