Warts are non-cancerous skin growths caused by a viral infection in the top layer of the skin. Viruses that cause warts are called human papillomavirus (HPV). Warts are usually skin-colored and feel rough to the touch, but they can be dark, flat and smooth. The appearance of a wart depends on where it is growing. There are several types of warts that can be treated in your physician’s office.
For more information: http://www.nlm.nih.gov/medlineplus/warts.html