Skin cancers fall into two major categories: melanoma and nonmelanoma
Melanoma can start in heavily pigmented tissue, such as a mole or birthmark, as well as in normally pigmented skin. It commonly appears first on extremities, chest or back, although it can occasionally arise on the palm of the hand; on the sole of the foot; under a fingernail or toenail; in the mucus linings of the mouth, vagina, or anus and even in the eye. Melanoma is a potentially aggressive, life-threatening cancer. It is usually curable if it is treated early. It progresses faster than other types of skin cancer and can spread beyond the skin to affect numerous parts of the body, including the bones or brain. Once this occurs, melanoma becomes very difficult to treat and is incurable.
Nonmelanomas are rarely life-threatening. These include basal cell carcinoma and squamous cell carcinoma. They progress slowly, seldom spread beyond the skin, are detected easily and usually are curable. In addition to the two listed above, there are a few rare nonmelanomas, such as Kaposi's sarcoma, a potentially life-threatening disease characterized by purple growths and associated with a suppressed immune system and almost always seen in patients with AIDS or the elderly.
Actinic Keratoses is a noncancerous skin growth that may become cancerous. These are crusty, reddish lesions that may scratch off but grow back on sun-exposed skin.