The three most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Each of these cancers develops in a different type of epidermal skin cell.
These cancers differ considerably in appearance. In all cases of skin cancer, however, early detection is the best chance for effective treatment. This is especially true in the case of melanoma, which spreads rapidly and has a high chance to invade other organs.
Basal Cell Carcinoma
Basal cell carcinoma is the most common of the three main types of skin cancer. In the United States, for example, more than 90% of skin cancers are basal cell carcinoma.
The cancer originates in basal cells in the skin, which are located in the bottom-most layer of the epidermis. Early basal cell lesions look like raised bumps on the skin. They are generally colored pink and are waxy in appearance. Often, these lesions bleed if they are scratched, bumped, or otherwise irritated. More advanced lesions look as though they have "sunk" into the skin. They are more likely to be flat, and may appear to be indented.
This type of cancer is the least dangerous of the skin cancers, and is rarely life-threatening. Basal cell carcinoma grows very slowly, and is unlikely to spread from its point of origin. Metastasis occurs in only 0.1% of cases.
Squamous Cell Carcinoma
This skin cancer is less common than basal cell carcinoma, but is more serious. Squamous cell carcinoma has a much higher chance of metastasizing, and can potentially spread to one or more organs. However, because this cancer spreads very slowly, diagnosis is usually made before it has spread further than the lymph nodes.
This skin cancer develops in squamous cells. These are scaly, flat cells that form the very top layer of the epidermis. Precursor lesions are called actinic keratosis. They appear as pink or red-colored patches on the skin, and are often rough or scaly-looking in texture. Actinic keratosis lesions have an approximately 1% chance of becoming cancerous.
Melanoma is the rarest skin cancer, but is also the most dangerous. Melanoma has a much greater tendency to spread than basal cell and squamous cell carcinomas. In the United States, melanoma diagnosis comprises only 4% of skin cancer diagnoses; however it causes 77% of skin cancer-related deaths.
This cancer originates in cells called melanocytes, which are the pigment-producing cells of the lower epidermis. Melanoma cancer typically starts as a dark brown or black lesion. It is irregular in color, and has an irregularly-shaped border. This cancer can develop on top of a pre-existing dark spot on the skin, or may develop in a location where no markings previously existed.
Cancer Research U.K.: Types of Skin Cancer
U.S. National Cancer Institute: What you Need to Know about Skin Cancer
University of California, San Francisco School of Medicine: Types of Skin Cancer
University of Maryland School of Medicine: Skin Cancer