Information from The American Cancer Society:

Normal skin

Skin is the largest organ in your body. It does several different things:

The skin has 3 layers (see picture):

Epidermis

The top layer of skin is the epidermis. The epidermis is very thin, averaging only 0.2 millimeters thick (about 1/100 of an inch). It protects the deeper layers of skin and the organs of the body from the environment.

The outermost part of the epidermis is called the stratum corneum, or horny layer. It is composed of dead keratinocytes that are continually shed as new ones form. The cells in this layer are called squamous cells because of their flat shape.

Living squamous cells are found below the stratum corneum. These cells have moved here from the lowest part of the epidermis, the basal layer. The cells of the basal layer, called basal cells, continually divide to form new keratinocytes. These replace the older keratinocytes that wear off the skin's surface.

Melanocytes, the cells that can become melanoma, are also present in the epidermis. These skin cells make the brown pigment called melanin, which makes skin tan or brown. Melanin protects the deeper layers of the skin from some of the harmful effects of the sun.

The epidermis is separated from the deeper layers of skin by the basement membrane. The basement membrane is an important structure because when a cancer becomes more advanced, it generally grows through this barrier.

Dermis

The middle layer of the skin is called the dermis. The dermis is much thicker than the epidermis. It contains hair follicles, sweat glands, blood vessels, and nerves that are held in place by a protein called collagen. Collagen, made by cells called fibroblasts, gives the skin its resilience and strength.

Subcutis

The deepest layer of the skin is called the subcutis. The subcutis and the lowest part of the dermis form a network of collagen and fat cells. The subcutis helps the body conserve heat and has a shock-absorbing effect that helps protect the body's organs from injury.

Benign skin tumors

Many types of benign (non-cancerous) tumors can develop from different types of skin cells.

Melanocytic tumors

A mole (nevus) is a benign skin tumor that develops from melanocytes. Nearly all moles are harmless, but having some types may raise your risk of melanoma.

A Spitz nevus is a kind of skin tumor that sometimes looks like melanoma. These tumors are generally benign and don't spread. But sometimes doctors have trouble telling Spitz nevi from true melanomas, even when looking at them under a microscope. Therefore, they are often removed, just to be safe.

Other benign tumors

Benign tumors that develop from other types of skin cells include:

Most of these tumors rarely, if ever, turn into cancers. There are a lot of other kinds of benign skin tumors but most are not very common.

Melanoma skin cancers

Melanoma is a cancer that begins in the melanocytes. Other names for this cancer include malignant melanoma and cutaneous melanoma. Because most melanoma cells still produce melanin, melanoma tumors are usually brown or black. But this is not always true, as melanomas can be non-pigmented (no color).

Melanomas can occur anywhere on the skin, but are more likely to start in certain locations. The trunk (chest and back) is the most common site in men. The legs are the most commonly affected site in women. The neck and face are other common sites.

Having darkly pigmented skin lowers your risk, but anyone can develop this cancer on the palms of the hands, soles of the feet, and under the nails. Melanomas in these areas represent more than half of all melanomas in African Americans but fewer than 10% of melanomas in whites.

Melanomas can also form in other parts of your body such as the eyes, mouth, and vagina, but these are much less common than melanoma of the skin.

Melanoma is much less common than basal cell and squamous cell skin cancers, but it is far more dangerous. Like basal cell and squamous cell cancers, melanoma is almost always curable in its early stages. But it is much more likely than basal or squamous cell cancer to spread to other parts of the body if not caught early.

Other skin cancers

Skin cancers that are not melanoma are sometimes grouped together as non-melanoma skin cancers because they develop from skin cells other than melanocytes. They tend to behave very differently from melanomas and are often treated in different ways.

Non-melanoma skin cancers include basal cell and squamous cell cancers (by far the most common skin cancers, and actually more common than any other form of cancer). Because they rarely metastasize (spread elsewhere in the body), basal cell and squamous cell skin cancers are less worrisome and are treated differently from melanoma. Merkel cell carcinoma is an uncommon type of skin cancer that is sometimes harder to treat.