This is a medical question of type : information 
 What is (are) Melanoma ? 
 Key Points
                    - Melanoma is a disease in which malignant (cancer) cells form in melanocytes (cells that color the skin).    - There are different types of cancer that start in the skin.    - Melanoma can occur anywhere on the skin.     - Unusual moles, exposure to sunlight, and health history can affect the risk of melanoma.    - Signs of melanoma include a change in the way a mole or pigmented area looks.     - Tests that examine the skin are used to detect (find) and diagnose melanoma.    - Certain factors affect prognosis (chance of recovery) and treatment options.
                
                
                    Melanoma is a disease in which malignant (cancer) cells form in melanocytes (cells that color the skin).
                    The skin is the bodys largest organ. It protects against heat, sunlight, injury, and infection. Skin also helps control body temperature and stores water, fat, and vitamin D. The skin has several layers, but the two main layers are the epidermis (upper or outer layer) and the dermis (lower or inner layer). Skin cancer begins in the epidermis, which is made up of three kinds of cells:          -  Squamous cells: Thin, flat cells that form the top layer of the epidermis.    -  Basal cells: Round cells under the squamous cells.    -  Melanocytes: Cells that make melanin and are found in the lower part of the epidermis. Melanin is the pigment that gives skin its natural color. When skin is exposed to the sun or artificial light, melanocytes make more pigment and cause the skin to darken.        The number of new cases of melanoma has been increasing over the last 40 years. Melanoma is most common in adults, but it is sometimes found in children and adolescents. (See the PDQ summary on Unusual Cancers of Childhood Treatment for more information on melanoma in children and adolescents.)
                
                
                    There are different types of cancer that start in the skin.
                    There are two forms of skin cancer: melanoma and nonmelanoma.   Melanoma is a rare form of skin cancer. It is more likely to invade nearby tissues and spread to other parts of the body than other types of skin cancer. When melanoma starts in the skin, it is called cutaneous melanoma. Melanoma may also occur in mucous membranes (thin, moist layers of tissue that cover surfaces such as the lips). This PDQ summary is about cutaneous (skin) melanoma and melanoma that affects the mucous membranes.   The most common types of skin cancer are basal cell carcinoma and squamous cell carcinoma. They are nonmelanoma skin cancers. Nonmelanoma skin cancers rarely spread to other parts of the body. (See the PDQ summary on Skin Cancer Treatment for more information on basal cell and squamous cell skin cancer.)
                
                
                    Melanoma can occur anywhere on the skin.
                    In men, melanoma is often found on the trunk (the area from the shoulders to the hips) or the head and neck. In women, melanoma forms most often on the arms and legs.    When melanoma occurs in the eye, it is called intraocular or ocular melanoma. (See the PDQ summary on Intraocular (Uveal) Melanoma Treatment for more information.)