Skin cancer is a disease where skin cells lose the ability to divide and grow normally. There are three main types of skin cancer - basal cell carcinoma, squamous cell carcinoma, and melanoma. Exposure to ultraviolet radiation from the sun is a major cause of skin cancer, along with genetics, radiation treatment, tanning beds, and sun lamps. People at higher risk include those with fair skin who spend a lot of time outdoors, have a history of sunburns, family history of skin cancer, many moles, or are over age 40. Skin cancer symptoms vary depending on type, but may include firm bumps or lesions on sun-exposed areas that may be flat, scaly, or change in color or size.