Warts are small, and non-cancerous growth that appears on the skin when the skin is infected with one of the many viruses of the human papilloma virus (HPV) family. The infection causes rough skin-colored bumps to form on the skin that looks like a solid blister or a small cauliflower. Warts most commonly appear on the hands, but they can also affect the feet, genitals, face, and knees.