The major source of ultraviolet (UV) radiation is the sun. Most of the light emitted by the sun is in the infrared and visible spectrum, but some of its light is in the ultraviolet spectrum (shorter wavelength than visible light). Other sources are for example tanning salons or even some light bulbs that emit small quantities of UV radiation.
While our body needs some UV to produce Vitamin D, too much of it is harmful to our body. An immediate effect of too much UV radiation are sunburns. In the long run, UV exposure increases the risk of skin cancer. We can protect ourselves against the adverse effects of UV by various protection measures such as not spending several hours on end in the sun or by applying sun screen. It is important to know that also in cloudy weather some UV radiation is not blocked by the clouds and reaches the earth’s surface.
Strictly speaking, part of the UV spectrum is ionizing as well; however, in epidemiology we commonly study health effects of UV radiation separately.
More information on UV-radiation and protection measures can be found here (in German)