Medical Encyclopedia by MedlinePlus, National Library of Medicine, U.S. National Institutes of Health. Start here for an introduction to skin cancer, the most common form of cancer in the U.S., and most prevalent after the age of 40 or 50. Explains the three predominant forms of skin cancer: (1) basal cell carcinoma, (2) squamous cell carcinoma, and (3) melanoma, which is the most dangerous form, with the greatest risk of spreading to other parts of the body, and the leading cause of death from skin cancer. Non-melanoma skin cancer has a relatively high rate of cure, if it is treated early. The principal cause of all skin cancers is exposure to sunlight.