Ultraviolet (UV) radiation is light (i.e., electromagnetic radiation) of a wavelength shorter than that of visible light, but longer than that of soft X-rays. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of visible light of shortest wavelength. UV itself can be subdivided into near UV (200-380 nm wavelength) and extreme or vacuum UV (10-200 nm).
Ultraviolet light is the cause of skin cancers such as melanoma. The radiation ionises DNA molecules in skin cells, causing mutations which can result in cancerous growths. As a defence to this the body tans when exposed to moderate (depending on skin type) levels of radiation by releasing the brown pigment melanin. This helps to block UV and prevent damage to the vulnerable skin tissues deeper down.
Ultraviolet lamps are also used in analyzing minerals. Minerals may look the same under visible light, but flouresce to different degress under ultraviolet light.