This is an example of a normal mole; note that it is almost perfectly round. Melanoma tumors differ in that they are usually asymmetrical (lopsided).
Although most moles are benign (not cancerous), certain types carry a higher risk of developing melanoma. About 2% to 8% of the U.S. Caucasian population has moles called "dysplastic" or "atypical" nevi, which are larger than ordinary moles (most are 5 mm across or larger), have irregular borders, and are various shades or colors. Individuals who have dysplastic nevi plus a family history of melanoma (a syndrome known as FAMM) are at an even higher risk for developing melanoma at an early age (younger than 40).