Skin cancer is the uncontrolled growth of abnormal skin cells. If left unchecked, these cancer cells can spread from the skin into other tissues and organs.

There are different types of skin cancer. Basal cell carcinoma is the most common. Melanoma is less common, but more dangerous.

The outer layer of skin, the epidermis, is made up of different types of cells. Skin cancers are classified by the types of epidermal cells involved:

• Basal cell carcinoma develops from abnormal growth of the cells in the lowest layer of the epidermis and is the most common type of skin cancer.
• Squamous cell carcinoma involves changes in the squamous cells, found in the middle layer of the epidermis.
• Melanoma occurs in the melanocytes (cells that produce pigment) and is less common than squamous or basal cell carcinoma, but more dangerous. It is the leading cause of death from skin disease.

Skin cancers are sometimes classified as either melanoma or nonmelanoma. Basal cell carcinoma and squamous cell carcinoma are the most common nonmelanoma skin cancers. Other nonmelanoma skin cancers are Kaposi's sarcoma, Merkel cell carcinoma, and cutaneous lymphoma.

Skin cancer is the most common form of cancer in the Unites States. Known risk factors for skin cancer include the following:

• Complexion: Skin cancers are more common in people with light-colored skin, hair, and eyes.
• Genetics: Having a family history of melanoma increases the risk of developing this cancer.
• Age: Nonmelanoma skin cancers are more common after age 40.
• Sun exposure and sunburn: Most skin cancers occur on areas of the skin that are regularly exposed to sunlight or other ultraviolet radiation. This is considered the primary cause of all skin cancers.

Skin cancer can develop in anyone, not only people with these risk factors. Young, healthy people -- even those with with dark skin, hair, and eyes -- can develop skin cancer.