Warts are skin growths that are caused by the human papillomavirus (HPV). There are more than 60 kinds of HPV, some of which tend to cause warts on the skin. HPV stimulates quick growth of cells on the skin's outer layer. In most cases, common warts appear on the fingers, near the fingernails, or on the hands. Certain types of HPV can also cause warts to appear in the genital area.