A nevus is a benign (non-cancerous) melanocytic tumor, more commonly called a mole. Nevi (the plural of nevus) are not usually present at birth but begin to appear in children and teenagers. Most moles will never cause any problems, but a person who has more than 50 normal moles (or more than 5 atypical or "dysplastic" moles) has a higher risk of developing melanoma, the most aggressive form of skin cancer.

normal mole
This gallery of photographs will alternate between normal, benign moles and melanoma so that you can learn to recognize each. Important note: Telling moles and melanoma apart is not easy, even for dermatologists with years of training, so be sure to contact your doctor if you have any questions.
The picture below of a melanoma skin cancer tumor shows how they are often irregularly shaped and multicolored.
The earlier melanoma is detected, the better the chance for successful treatment. Monthly self-examinations may help find it early. Often, the first sign of melanoma is a change in the size, shape, or color of an existing mole. It also may appear as a new or abnormal-looking mole. The "ABCDE" rule can be used to help remember what to watch for.

Melanoma Skin Cancer Moles Part II
Melanoma Skin Cancer in Depth is HERE
SKINIPEDIA, your Skin Encyclopedia