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.
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.