Skin moles, also known as nevi, are a common type of skin growth that can occur anywhere on the body. Most people have moles, and many people have between 10 and 40 moles by adulthood [source: WebMD]. Skin moles usually appear brown or black in color, and they can be any shape or size. Each mole has a different growth pattern, so a mole may change over the course of a person's life. A shift in a mole's appearance isn't, in itself, a cause for concern.
Generally, a mole begins as a flat, tan spot like a freckle on the skin. As a person ages, the mole can become more raised and its color may change, or the mole may disappear or fall off. These are natural changes in the life cycle of a mole, which may last as long as 50 years [source: American Academy of Dermatology].
Two types of moles are more likely to present problems later in life: congenital nevi and dysplastic nevi. Congenital nevi are moles that are present at birth. They occur in about one in 100 people [source: WebMD]. These moles may be more likely to develop into melanoma than moles that occur after birth, so it's important to watch for significant changes.
Dysplastic nevi, or atypical moles, are larger-than-average moles that have an irregular shape. They have uneven coloring, usually with dark brown centers and lighter, jagged edges. Dysplastic nevi may also have a reddish border or small black dots around their edge. People with dysplastic nevi often have 100 or more moles. These moles tend to run in families and can also pose a risk of melanoma [source: American Academy of Dermatology].
Being able to recognize the different types of moles is important for conducting self-examination, which can help individuals identify moles that may develop problems later.
So what causes moles, anyway?