The name melanoma comes from melanocytes, which have to do with pigment in the skin (that gives us our coloring). Melanocytes often proliferate, that appear on the skin's surface as small, dark, flat, or dome-shaped spots, which are usually harmless moles or liver spots.

But when those pigment cells grow out of control, then you have melanoma.

Here's a list of things to look for to determine if a dark spot is a harmless mole or the start of melanoma:

• Asymmetry (A). About half the time, a melanoma develops in an existing mole; in other cases, it arises as a new lesion that can resemble an ordinary mole. A noncancerous mole, however, is generally symmetric and circular in shape, while melanoma usually grows in an irregular, asymmetric fashion.
• Border Irregularity (B). Benign lesions generally have clearly defined borders that mark the boundary between mole and skin. A melanoma, in contrast, often has notched or indistinct borders that may signal ongoing growth and spread of the cancer.
• Color Variation (C). One of the earliest signs of melanoma may be the appearance of various colors within the lesion. Because melanomas arise within pigment-forming cells, they are often varicolored lesions of tan, dark brown, or black, reflecting the production of melanin pigment at different depths within the skin. Occasionally, lesions are flesh colored or surrounded by redness or lighter areas of depigmentation. Pink or red areas may result from inflammation of blood vessels within the skin; blue areas reflect pigment in the deeper layers of the skin; and white areas can arise from dead cancerous tissue.
• Diameter (D). A diameter of 6 millimeters or larger (about the size of a pencil eraser) is worrisome. Melanomas start out small; by the time a lesion has grown this large, other abnormalities will most likely be present. No matter what size, a doctor should examine any suspicious lesion.
• Evolution (E). A lesion that is growing or changing deserves evaluation.

Click here for a list of tips for checking moles and monitoring your skin for melanoma.

Click here for a list of tips for preventing melanoma. Most of it has to do with general sun safety:

• Use sunscreens that block out both UVA and UVB radiation.
• Wear sunglasses and clothing that covers you up.
  
• Avoid exposure particularly during the hours of 10 AM to 4 PM, when sunlight pours down 80% of its daily UV dose.
• Avoid reflective surfaces, such as water, sand, concrete, and white-painted areas. (Clouds and haze are not protective and in some cases may intensify UVB rays.)
• Ultraviolet intensity depends on the angle of the sun, not heat or brightness. So the dangers are greater the closer to the summer-start date. For example, in the Northern Hemisphere, UV intensity in April (two months before summer starts) is equal to that in August (two months after summer begins).
• The higher the altitude the quicker one sunburns. (One study suggested, for example, that an average complexion burns at 6 minutes at 11,000 feet at noon compared to 25 minutes at sea level in a temperate climate.)
• Avoid sun lamp, and tanning beds or salons. They provide mostly high-output UVA rays. Some experts believe that 15 to 30 minutes at a tanning salon are as dangerous as a day spent in the sun. 

See also:

Rheumatoid Arthritis Drug Boosts Melanoma Incidence

Painkillers don't protect against melanoma