Basal cell carcinoma is the most common form of skin cancer. It is a slow-growing cancer that occurs mostly in sun-exposed areas of the body, although it can occur in parts of the body that aren't exposed to the sun, and rarely if ever spreads beyond the skin. Squamous cell carcinoma is the second most common form of skin cancer. It also occurs mainly in sun-exposed areas. Unlike basal cell carcinoma, squamous cell carcinoma can occasionally spread to the lymph nodes. Anyone who has had one tumor is also at increased risk of developing another.