Moles are a type of nevus. A nevus is an overgrowth of normal tissue. You can get a nevus of different tissues such as a nevus of hair (called a hairy nevus) or a nevus of blood vessels (called a hemangioma or vascular nevus), and in fact there are many types of nevi. A mole is a nevus of nevoid cells or pigment producing cells. The term for a mole in medicine is a nevoid nevus which is an unfortunate name because it starts to sound confusing. The vast majority of moles are harmless. However, moles can become melanomas. While most moles do not become melanomas, it is generally considered true that about 20 percent of melanomas come from moles. The other 80 percent of melanomas probably start as melanomas right from the beginning. If you have a mole that is changing in any way, such as recent itching or shape or color change, be sure to get it checked by your doctor.