Cancer develops when DNA, the molecule found in cells that encodes genetic information, becomes damaged and the body cannot repair the damage. These damaged cells begin to grow and divide uncontrollably. When this occurs in the skin, skin cancer develops. As the damaged cells multiply, they form a tumor. Three types of skin cancer account for nearly all diagnosed cases. Each of these three cancers begins in a different type of cell within the skin, and each cancer is named for the type of cell in which it begins. Skin cancers are divided into one of two classes: non-melanoma skin cancers and melanoma. Melanoma is the deadliest form of skin cancer.